Absolutely better regs around access to diagnostics (e.g. CAN bus as you mention) is part of the story. But for the bigger internal stuff, tolerances are a lot tighter than they were 20 years ago, which can make it hard to do a good job of in your driveway. Certainly some superficial repairs are made much harder than they should be, but a lot of that is more modular too so you aren't saving a ton doing it at home. At least you can find out which control unit should be replaced if you can read the diagnostics properly :)

Interesting point about the tolerances, hadn't though of that. For making your own machined parts (eg crankshaft, engine block/heads), that makes sense.

Though, making your own engine block is pretty far from a "home garage thing" anyway.

People used to just buy those parts from a local supplier or wrecker, then fit them in their garage at home. That doesn't seem like it should have changed.

This already exists, OBD2 is a standard diagnostics interface mandated by law. I would guess most of the tools you are referring to are using this.

Yeah, but AKAIK the codes themselves seem to often be manufacturer internal info.

People into automotive (and boat, uav) CAN stuff have figured out ways to reverse engineer much of it though.