Given that I'm much less experienced than most players here, let alone on MafiaScum or MafiaUniverse, my opinions might not be worth much, but I'll give them anyway.
I disagree that alignment-changing roles are necessarily bastard. They're definitely not bastard if the possibility of alignment change is publicly announced by the start of the game. I'm used to playing with cults and recruiters because those are common in my Discord playgroup, and I'm not familiar with the gameplay cultures of MS/MU, but I'm guessing that they consider these bastard because their metagames evolve over geologic time and cults were seen as against the spirit of Mafia in decades past.
Scum editing role flips, i.e., Tailor, is borderline bastard. The problem with requiring games with this to be declared bastard is that announcing its existence makes it much weaker.
Directly lying in role PMs is definitely bastard. Hidden mechanics, which my upcoming Culture game will have, are not always bastard. I wouldn't call it bastard if someone had a hidden Bulletproof, but a hidden Paranoid Gun Owner is 100% bastard. I do think it would be bastard for role flips to not reveal all hidden mechanics.
My tentative proposal is that we compile two lists of bastard mechanics: ones that are totally bastard, like outright lies in role PMs; and ones that are kinda-bastard, like cults and Tailor. These lists might never be comprehensive, nor should they aim to be. Moderators can use the lists to decide whether their game is not bastard, kinda-bastard, or totally bastard, with games being classified by their most bastardy element. Mods can declare their game as any level of bastard or say nothing. Whether players should refuse to play in games where the mods do not reveal the bastardization quotient is something I think we can figure out over time.