Rather than how complete it is, the real improvements of the proposal are:

1. it all but requires that ES-defined functions stringify to their source code. Pre-ES2019 that's implementation-defined

2. it standardises the placeholder for the case where toString can't or won't create ECMAScript code (e.g. host functions), this could otherwise be an issue as with implementation-defined placeholders subsequent updates to the standard might make the placeholder unexpectedly syntactically valid, by having the placeholder standard future proposals can easily avoid making it valid

3. the stringification should be cross-platform as the algorithm is standardised

If you are interested in all the gory details, take a look at the TC-39 proposal (especially the Goals section) and associated GitHub issues:

https://tc39.es/Function-prototype-toString-revision/

https://github.com/tc39/Function-prototype-toString-revision...

I don't know the answer to your question, but I would like to add to it to gain some clarity for myself: why not give the caller the option to have comments eliminated? An optional parameter `includeComments` bool with default `false` would provide backward compatibility while allowing those who need the comments to request them.