Monorepo doesn't "work around" it, it means you have to support everything (including migrating/BC) on the current version, instead of having to support old stuff on the old version.

You can see where the “library” is used. Often the bit you want to change is only used by your project so you can change it freely. If someone else is using it you know who to talk to about changing it.

You can “fork” inside the monorepo too, just take a copy.