How to deprecate and/or remove unused runtime and JS library functions?

We have had some issues over there years where emscripten itself stops using a runtime or JS library function and we would like to remove it because its now dead code and untested.

See the discussion on https://github.com/emscripten-core/emscripten/pull/14009.

It would be nice if we could find a way to avoid this in the future. I see two ways to improve the situation.

## Avoid exposing internals going forward

The runtime and JS library system doesn't really have a good way to avoid exposing all of its internals to the user-level library.js code and EM_ASM code. Can we find a way to expose less to the user's library code so we can iterate on internals with impunity?

## Create some kind of deprecation path

When a runtime function is up for removal/deprecation it would be good to have a plan for how long to keep it alive, and how to keep it tested.