Is it true that black holes emit only photons, electrons, and positrons in the form of Hawking Radiation? According to this comment, if the peak photon energy is 511 KeV, electrons and antielectrons begin to be produced along with photons. So when the peak energy is ~940 MeV, the rest mass of a proton, will they be produced as well, and what types of subatomic particles can be generated via Hawking Radiation?
When you look at Hawking's 1974 Nature paper (https://www.nature.com/articles/248030a0), it is very clear that like thermodynamic blackbodies, black holes can emit anything they want. Or to be more precise, the thermal radiation that is produced by gravitational vacuum polarization in the vicinity of the black hole event horizon is not restricted to photons.
But what else could it be?
Photons are there, obviously. And their peak wavelength is determined by the temperature of the black hole or blackbody. Unless that temperature is several billion degrees kelvin, there just isn't enough energy to form electron-positron pairs in meaningful quantities. (Astrophysical black holes have temperatures in the nanokelvin range.) Neutrino-antineutrino pairs perhaps, but the extreme short range of the weak interaction effectively limits their radiation. Gravitational thermal radiation (assuming gravity is quantized and gravitons exist - see Dyson's Poincaré prize lecture, section 5 in particular: https://publications.ias.edu/sites/default/files/poincare2012.pdf ) might also be present but again, close to twenty orders of magnitude less intense than photons.
If the temperature is high enough for electron-positron pairs to form, sure, those can also come out of the black hole. As well as proton-antiproton or neutron-antineutron pairs. But these hot black holes would be so small (just a few billion metric tons, with Schwarzschild radii in the subatomic range), they would have to be either primordial black holes or astrophysical black holes near the end of their evaporation, in the extreme distant future.
What else? There are no other stable particles for the black hole to emit. If a sufficiently hot black hole produces, say, a W+/W- pair, those would very rapidly decay the usual way into positrons (electrons) and electron (antielectron) neutrinos, or their muon/tau flavor counterparts (but those, too, would decay.)
So in the end, just like in the case of a thermodynamic blackbody, we find that although in principle, a black hole can emit other things, at non-extreme temperatures, the only thing it can emit with any efficiency is photons.