We're in the process of updating our documentation to help people port their existing plug-ins and scripts to 3.0: https://testing.developer.gimp.org/resource/writing-a-plug-i...

Now that the API is stable for 3.0, we're hoping more people will feel comfortable updating to it.

Gimp 3.0 is still in release candidate and nobody is forced to move to gimp 3.0 until their favorite plugins are ready.

Additionally it is trivial to have both installed on the same machine and there haven't been breaking changes in the xcf format as far as I know so you can open the same file on both version without issues.