The original argument for supporting mapping of URLs was to enable polyfill workflows where https://path/to/builtin-polyfill.js could be mapped to the builtin with a fallback.

The reason this was necessary was to support both browsers without import maps support and browsers with import maps support for builtin shimming.

With the newer timelines we are likely looking at import maps shipping quite a while before builtins ship, in which case this constraint no longer applies.

Since this constraint that initially pushed for URL mapping is no longer there, do we still need to support URL mapping?

The alternative is to just support bare specifiers in import maps.

Personally I prefer this for consistency of resolution and in knowing that eg import(await import.meta.resolve('pkg')) is always well-defined, instead of possibly resolving differently to import 'pkg'.