Is there a way to make the specific behavior more useful to others - making the specific behavior more a specific configuration of a more broadly generic extension?
Cf what I was mentioning in Upstreaming, forks, plugins & themes about the use of plugins:
Also note that even in the case of a plugin, we should still contribute it to the project: post product proposals, ADR, and if there is interest for having it in the standard distribution, still get it included there. This is important, as more and more features will be developed as plugins - we should still do the effort of “upstreaming” those.
It is one of the 3 big no’s: “Making the change in a plugin that we don’t attempt to contribute to the project” - it might be in a plugin, but it still deserve the same attention as any code to upstream.