You’re conflating a major engine version change with point releases. I wouldn’t expect an Unreal Engine 4 to work with Unreal Engine 5 in the future. Unity 4 plugins may not work with Unity 5. However Unity 4.X plugins almost always work when Unity upgrades to 4.Y.
A point release from Unreal 4.X to 4.Y should endeavor to maintain API parity using deprecation, just like Unity 4.X to 4.Y does and this would solve many of the issues.
BTW: RadiantUI provided the full source code to licensees. I don’t know what your particular plug there is with it but in the context of this thread that doesn’t make sense since you got the full source code.