Hypothetically though
Can't a woman who took testosterone
Just take estrogen, and it reverses the changes?
No. I know Logiko will attack me for this - but that is patently false. I know many detrans females to males who have irrevocable vocal chord damage by testosterone. The throat itself is not always wide enough for the changes of male puberty, causing permanent, chronic pain without surgical intervention to those whom this applies (which isn't always successful and never returns the natural voice - note, this is not a standard treatment, and most detrans women cannot get this due to the absurd costs).
Whilst not all FtMs lose their fertility, some do due to the masculinising hormones. There are changes that are more permanent as a result of the masculinisation process, from impacts to their hairline all the way to their facial appearance (though some of it can be feminised). The degree of reversibility runs parallel to the amount of time they were transitioning.
Then you need to consider MtFs. Breasts cannot be removed without surgical intervention. Virility is almost always permanently impacted should they be on estrogen long enough, and especially if they use puberty or testosterone blockers. In terms of reversibility of feminine appearance, quite a bit can be changed with testosterone, but as illustrated before, not all of it.
This all excludes surgical transgender medical care. A man who undergoes a vaginoplasty will never have his penis back. A woman who has a masectomy will never have functional breasts for child-rearing. A woman who has a hysterectomy is not getting her ability to birth children back. Facial feminisation surgery is not really reversible, and neither is vocal feminisation surgery.
It's not as simple as 'all can be reversed'. Anybody who tells you that is misinformed, an zealous activist, or a malicious actor.