RFCs is another type of technical decision making documentation that has been popular for decades now, and can probably be found more widely as it's more spreadout online from what I can tell. It's worth pointing out that RFCs are less easier to locate it next to specific implementations, since it focuses heavily on a more abstract type of standardization and specification document, and lives further away from the code than ADRs for example.
All of that to say if the intent is to track down code historial evolution, some types of RFCs out there _might_ help shed some facet of the light you may be looking for.
And GitHub too for that matter - e.g. "stars:>100 forks:>20 path:/adr license:apache-2.0" this will search for GitHub repos which have over 100 stars, over 20 forks, Apache 2.0 OSS licence, and a directory named "adr"