It’s up to the plugin/theme author here. Either they can continue with the current strategy, where the main branch of the plugin needs to work with all ‘released’ versions of Discourse. Or they can use the automatic branching strategy, which makes compatibility easier, but also means you may need to do a lot of ‘backporting’ in your plugin in case of any critical bug/security fixes.
At any given time, there are three “supported” versions of Discourse, plus latest. So critical fixes will need to be applied in up-to 4 branches.