Another non-exact answer, though I thought I'd add some color of a different shade: ADR is a relatively recent concept that's been making its way into organizations and becoming more mainstream that its "grandfather" - or in nicer terms, counterpart - RFC (Request For Comments).
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.
