Non-type-related issues for v7 beta

[cartant]

This issue is for tracking non-type-related issues - type-related issues are in #5066 - that could/should be addressed in the v7 beta.

• Use hostReportError for swallowed errors - Exception not thrown when maximum call stack is exceeded #5370 (From @benlesh: Here we opted to add a skipped test that we can "unskip" when we get to the larger refactor in v8)
• console.warn still seems to be used instead of hostReportError - see this comment
• Several issues related to the behaviour of notification observables that complete - delayWhen should use an indefinite delay for notifiers that complete without emitting #3665, takeUntil exhibits weird behavior when notifier is a previously completed connectable observable #3853, skipUntil(s$) doesn't complete when s$ completes #5379 - these (and the documentation) should be made consistent. I suspect that audit has this problem, too. All operators that have a notifier need to be checked. (the only remaining PR on this one is here: fix(delayWhen): no longer emits if duration selector is empty #5769) Nope, there are a whole bunch of operators that treat next and complete identically in notifiers/selectors. See Complete notifications deemed to be signals #5838 and fix: don't treat notifier completion as a signal #5853 (the PR)
• mergeScan has strange behaviour upon the completion of the inner observable - see here - this should be changed. (PR here: fix(mergeScan): No longer emits state again upon completion. #5805)
• animationFrames should use the API's DOMHighResTimeStamp - animationFrames should emit the timestamp from requestAnimationFrame's callback #5194
• finalize is dodgy AF - see Finalize is not called at end of pipe, different results with delay #5357 and issues linked therein.
• Subscribable sources without Symbol.observable - see from is typed to accept any Subscribable, but only accepts Observables with Symbol.observable #4532
• Scheduler exports - see Make schedulers compatible with babel-plugin-transform-imports #5421
• @internal - see Mark publicly-visible internal properties as @internal #5768
• We should document that the minimum TypeScript lib for RxJS is ES2018, as that's when async iterables were introduced. That issue is that, our type inference tests assume that the types in ES2018 are available and weird things can happen if they are not. See Impose a minimum TS lib for version 7. #5708. chore: add triple-slash ref to asynciterable #5807 adds a workaround, but I think it's still worth documenting. Maybe reference this blog post? Not sure where this would go in the docs.
• user-land operator subscriber - see Export an operator subscriber for user-land operators and refine Observable ctor #5836

Do Not Block The Release

• concurrency parameter for conactMap - see Allow concurrency in concatMap #5519 (mergeInternals could be extended to take a concat or collate flag)
• deprecations (breaking change documentation DOES block the release, but not the deprecations, as they're non-breaking): Create documentation for deprecations and breaking changes and add URLs messages #4776; About consistency, deprecations and docs #5404

[benlesh]

finalize isn't really an issue. Just a confusing outcome from the implementation fo concatMap and a synchronous inner observable.