> https://github.com/tc39/proposal-canonical-tz - appropriately to these comments, a proposal to handle tzdb changes, built on top of JS Temporal, includes some great examples of all the ways this can happen

Thanks! I was the co-champion of that proposal. Parts of it were merged into Temporal last year, and other parts are already part of the JS Internationalization (ECMA-402) specification here: https://tc39.es/ecma402/#sec-use-of-iana-time-zone-database