We've been capturing those decisions in ADRs (https://www.thoughtworks.com/radar/techniques/lightweight-architecture-decision-records https://www.thoughtworks.c

We've been capturing those decisions in ADRs (https://www.thoughtworks.com/radar/techniques/lightweight-architecture-decision-records https://www.thoughtworks.com/radar/techniques/lightweight-ar...) for the past year or so, and they've been incredibly helpful even for those involved in the original decisions that want to revisit the rationale - I can only imagine how helpful they'd be in a situation like yours.