Deprecate Reopens After Instantiation

[chadhietala]

I've mentioned this before in https://github.com/emberjs/ember.js/issues/12200, seems like we may want to do this in the new object system. While I understand that it may hinder addons, it will be more of a foot gun when async engines are a thing and you lazy load some code that potentially modifies the prototype of all the classes already held in memory.

Don't want to get into scope creep, but I think this needs to be done regardless of new object system.

[sandstrom]

I'm doing some issue gardening 🌱🌿 🌷 and came upon this issue. Since it's quite old I just wanted to ask if this is still relevant? If it isn't, maybe we can close this issue?

Since classes (modern JS) are no longer reopenable, maybe this isn't an issue anymore?

By closing some old issues we reduce the list of open issues to a more manageable set.