Personally I would say “meh” about working too hard for Julia versions prior to 1.9. Packages should function correctly on all Julia versions, but if a recent Julia version provides the best experience I don’t think we need to kill ourselves trying to match it on older versions, since we can just tell people to use the latest release. (Yes, I know 1.9 is not yet released…)
If you want to make changes that optimize the 1.9+ experience but significantly degrade the experience on older Julia versions, to me that sounds like time for a new minor (or breaking, if necessary) release and minimum bounds on the Julia version. In weird cases you might need RetroCap or something. That will allow you to keep users on the older versions until they adopt a Julia version optimized for the new design.
In my view we should play the long game, rather than going to a lot of effort to make improvements that will have a transient lifetime.