Strongly recommend this plugin. The formant pitch shifting is pretty much the only way I can get a good femme vocal recording from my testosterone laden cig-y vocal chords. The rest of the Soundtoys collection is pretty fantastic, but replacable. Alterboy is the only plugin I know of that lets you pitch shift the way it does.
Is there one that makes femm voices sound more masc?
Alterboy will do this yeah! Or as other commenters have mentioned, there are other formant shifting plugins, Alterboy is just the one I've been using for 4+ years. When I do vocal mixing, I often add it in small doses to already masc voices to add more grit. You also hear it a lot on Brent Faiyaz's vocals, his engineer LOVES formant shifting. You just pull the knobs down instead of up.
Basics of formant pitching so it makes sense why it's a big deal for trans people. Normal pitching is just slowing or speeding up a sound. I think this is why we see so many trans people in PC music and Hyperpop, the speed change pitching done out of necessity (aka dysphoria) from older musicians just became a defining part of the sound. Formant shifting takes the peak frequencies of the sound, the formant, and will adjust those to change the pitch instead. It can be used for a variety of different sound effects. The best example I can give for the masc-ification of Alterboy is Story of OJ by Jay Z where the formant shifting is used on the vocal sample to both raise and lower the pitch. The part where the sample says "black... is yellow" is a great use of making a femme performance more masc.
That rules, thank you