nit: I think we can swap the name of ts37.d.ts with index.d.ts. So index.d.ts always align to latest TS, and we can have lib/legacy.d.ts as fallback.

{
  "types": "lib/legacy.d.ts",
  "typesVersions": {
    ">=3.7": {
      "*": "lib/index.d.ts"
    }
}

The thing is, importing lib/index.js directly would cause old TypeScript versions to try to load lib/index.d.ts, but that would then have the TypeScript 3.7 syntax.

Using index.ts37.d.ts also ensures that we can easily add index.tsXY.d.ts (e.g.: index.ts40.d.ts) in the future.

We will disable submodule imports in Babel 8. And we could drop ts <3.0 support, too. So tsc will always respect typeVersions and types and we can enforce that lib/index.d.ts always aligns to latest TS while typeVersions serves a fallback for older ts>=3 versions.

typesVersions is only supported since TypeScript 3.1.

Also, this will need to be kept as index.ts37.d.ts in Babel 7.