At the top, you have the true standard library for the language. This has very strong stability guarantees. Its purpose is twofold: to provide universal implementations of essentials and to define standard/baseline interfaces for common needs like abstract data types, relational databases, networking and filesystems to encourage compatibility and portability.

Next, you have a tier of recognised but not yet fully standardised libraries. These might be contributed by third parties, but they have requirements for identifying maintainers, appropriate licensing and mandatory peer review of all contributions. They have a clear versioning policy and can make breaking changes in new major releases, but they also provide some stability guarantees along the lines of semver and older releases are normally available indefinitely. The purpose of this tier is to provide a wider range of functionality and/or alternative implementations, but in a relatively stable way and implementing standard interfaces where applicable to improve portability.

Finally, you have the free-for-all, anyone-can-contribute tier. This should still have a sane security model where people can’t just upload malware scripts that run automatically just because someone installed a package. However, it comes with few guarantees about stability or compatibility, except that releases of published packages will be available indefinitely unless there’s a very good reason to pull them where you obviously wouldn’t want to use one anyway. A package you like might be written by a single contributor who no longer maintains it, but if someone does write something useful that simply doesn’t need any further maintenance once it’s finished and does its job, there is still a place to share it.

PHP is a fantastic resource to learn how to do proper backward compatibility and package management. By doing the exact opposite of whatever PHP does, mostly.