(IANAD) A trans woman with neither estrogen nor testosterone in her system wouldn't begin to masculinize, but she would experience menopause-like symptoms such as hot flashes or risk of osteoporosis. This is why generally even after orchiectomy or SRS, trans women will still take estrogen for their entire lives, even though they no longer need to take an antiandrogen to prevent masculinization.

If a trans woman does have testes, and stops taking both estrogen and the antiandrogen, then she will gradually begin to masculinize. My understanding is that trans women will sometimes stop taking hormone therapy for up to several months for things like banking sperm, so it's not like forgetting to take your hormones for a few days is a major crisis. But going without hormones for extended periods of time is also not particularly fun.

That said, there are trans people who never take hormones, or who stop taking hormones, or who temporarily or permanently lose access to hormones, for any number of reasons, and their transitions aren't any less transitions for it. For trans men with CAIS, for instance, taking testosterone would have no effect on their bodies, but this doesn't make them any less men.