This is gonna get long, but I'll try to highlight the most important stuff and what you ask about specifically. I'm in the same boat as you, was on T for 6 years and detransitioned. I stopped taking it around 7 months ago. My T levels are almost back down to female range now (last I checked they were just slightly above and that was a few months ago, so by now I think they probably are in female range) but my estrogen has barely kicked in at all. It's actually at rock bottom, just shooting a few lame sparks, pretty much. My hormones all being that low for several months made me feel really terrible with unbearable menopausal symptoms, so I got put on low dose estrogen while waiting for my ovaries to wake up from their T-coma... if they even will. I dunno, only time will tell, as I can only keep checking up on them and their productivity every once in a while.
As far as I know, it takes around 6 months to a year for the extra testosterone to leave the body entirely, after having stopped taking it. The estrogen however... it's way too uncertain and too individual to even guesstimate, really.
Not a whole not has reversed since I went off T but here's what has reversed:
- Body odour changed back to a more female smell quite fast, within the first few weeks.
- Fat distribution is slowly on its way back to how it was before, but not yet entirely reversed. Although I should say on this point that the fat in my face never actually changed on T to begin with, so that's not gonna change going off T either.
- Periods are back, but very irregular and sometimes with horrible, horrible cramps. I've only had 3 periods since I went off the T, one of which lasted for 26 whole days (yes, an entire month of bleeding! Thanks I hated it), the other two for around 14 days (2 weeks) each. But I'm sure that mess will stabilse itself eventually.
- Skin has softened remarkably. Also no hormonal acne this time!
- Clit has shrunk a little (by about 20%).
- Body hair har reduced a little bit, and I mean little. I only see difference in my leg hair, nowhere else, and I'm Hairy with a capital H.
And here's what has not reversed at all at this point:
- Voice is still exactly as deep as it once became. However some people have told me it sounds higher but that's just in their heads. I did check my pitch again rather recently and it's still at 109hz on average (I used the pc program Praat). However I do speak with more melody since detransing cause I'm not worried about not "sounding like man" anymore, which has nothing to do with my hormones. So my speech pattern is more feminine, but my pitch is not.
- Facial hair is just as thick, dark and merry as ever. Only difference there is it now hurts to shave cause of my skin being softer and more delicate again. I nick it so easily now, and it gets red and sore no matter how gently I shave.
- All the hair on my upper body is the same, on chest, stomach, arms, and even the few stray hairs I acquired on my back shortly before I went off T.
- Hairline is still the same strongly "M" shaped situation, which can surely be seen in my profile pic, that shining white high temple.
So that's what's been happening with me. Here's what I know about what can reverse or at least reduce after a longer time of not having taken T (at least a couple of years off it):
- Body hair can reduce, so it's "semi-permanent." Some only get a little reduction, others get almost complete reduction. This seems to be less likely the longer you've been on T, but I dunno what exactly that means in terms of time.
- Fat distribution, muscle mass, skin softness and body odour are entirely reversible. Fat re-redistribution for me meant my waist is getting slimmer again, and the butt/hips of doom are back. I never gained any noticable muscle mass though, cause I'm a couch potato. So I don't have any experience with losing muscle either.
- Excessive sweat due to T will reverse too. As for me I've been an excessive sweater since I was a tiny teen, and this fact didn't change in either direction whether on or off T. So it's still just as excessive, but also was not caused by the T to begin with.
- Facial hair can and will most likely thin out over time and grow more slowly, but it won't disappear entirely, unless you only got a few strands and/or peach fuss. If you'd want it gone then you'd have to go for laser or electrolysis. Although, to give a little personal detrans advice, I'd be careful to really think that through before going for permanent removal if you're considering it. Cause that kind of removal REALLY is permanent!
- Voice won't reverse at all. Although I've heard a few say their voices got slightly higher again, I'm doubting that's factual. However if you want your voice to sound higher and/or more feminine again, there's voice training. Quite a lot of tutorials of that on youtube, and some demonstrate really good techniques, all for free. I tried it out a little until I got disinsterested and fell in love with my deep voice again. But still.
- Clit growth can shrink a little but not entirely reverse.
- Lost hairline may grow back in to some degree, depending on how recently you lost the hair, but I'm unsure about this. I'm not a huge fan of my own hairline, but ultimately I accept it so I haven't looked much into this.
- Periods will likely return but there's a possibility they might not.
As for psychological changes your mood will crash at first, but even out eventually. Being able to cry again, connecting better to your emotions, etc will likely happen. Meaning psychological changes do reverse as well, but need some time to stabilise first. That took a few months for me. The first 2 months off T for me was a hellish roller-coaster, but then it got significantly better. I did go off it supervised and gradually, but a temporary mood dip is probably inevitable no matter how you go about it. It will pass though.
The one thing I'm really unsure of is cartilege growth. You know, that "half hard" rubber-like tissue in your ears and nose tip? Yeah that stuff, cause there's a lot of that in the jaw area too which grows when you take T and is what gives a more masculine looking face (aside from also fat redistribtion does that, but which I personally for whatever reason didn't get). It's also the reason sometimes hands and feet increase a size or two as well. I've no idea if grown cartilege can reverse or not, but my wild guess is that it probably can't.
As for your head/skull size:
Actual bone growth though, that doesn't happen on T, unless you take it before your growth plates fuse, which they do around roughly age 20. So I don't think your skull could have grown, but IF it did... sorry, you're stuck with that. Buy a bigger hat!
As for reproduction, I've definitely heard of trans men who've been on T for 3, 4 or even 5 years go off it and then successfully carried a child to term, with their own eggs. But long term use of T can also definitely reduce fertility long term. If you get your periods back there's a chance you could still be fertile, but not certainly. I've heard some who go off T after a long time on it (+5 years) stay in menopause and don't come out of it again. They are then infertile.
That's about as comprehensive as I can make it, I think. With both my own personal experience of going off T so far and what I know about it in general and can expect for myself. But yeah, there is very little research about this and your best bet for info is anecdotal info. But also do please keep track of your hormone levels, especially if you're starting to feel bad or something strange happens. Sometimes your body needs a little help finding back to its natural levels, and sometimes it can't. So do listen to whatever it's telling you. And also please don't rush into reversing stuff with laser or surgery, etc, if you're considering it. Give it a year at least. That was one of the best (and most annoying!) detransition advice I was given early on after I had realised my situation.
