Looked very simple. Had a relay for the oven and 2 temp sensors. One for controlling the oven, and another to shut everything down if the temp outside the oven got too hot.
I do wonder how many types of these boards would be required to fit the common appliance types.
Probably the biggest new difference I've seen is brushless motors with variable frequency drives, which requires its own power control electronics and probably some calibrations.
So in my estimation not very many. Packaging it all up in a way that an appliance tech could be comfortable installing would be the hard part.
Documentation would be fantastic, but it feels like a curation/discovery problem. After a bunch of deep searching, you can usually find a forum where people are discussing various lines of machines at a higher level, what features they have, repairability, etc. Parts websites generally have diagrams and whatnot (no idea where these come from or what it takes to get access to it wholesale). You can usually figure out cross reference part numbers from pouring through eBay/Amazon listings. And sometimes you even stumble onto a wiki or something where someone has documented a specific machine tangential to another project (eg Home Assistant).
Gathering it all up in one place to be useful, especially in a way so people will contribute their findings back so it can be self-sustaining, seems like the hard problem to me.
I feel like retrofitting older appliances might be the way forward, whether preemptively or as part of a repair. Although I do admit there are benefits to modern brushless drives (etc), that would be hard to design a generic control board for.
Although recently having gotten a tiny bit of experience with off the shelf PLCs, going that way might be the right answer rather than reinventing the wheel.