Lightweight ADRs are a good recommendation. I've put similar practices into place with teams I've worked with. Though I prefer to use the term "Technical Memo", of which some contain Architectural Decisions. Retroactive documentation is a little misaligned with the term ADR, in that it isn't really making any sort of decision. I've found the term ADR sometimes makes some team members hesitant to record the information because of that kind of misalignment.

As for retroactively discovering why, code archeology skills in the form of git blame and log, and general search skills are very helpful.