I’m very sorry to hear that!
If you do want to proceed down this path, I’d recommend adding a note to the README & Meta topic, marking it broken and then archiving the GitHub repository. That way, it’s still available for others to fork (assuming the license is permissive enough).
We absolutely do care about compatibility & keeping things working. That’s why we have these long deprecation periods with fully automated upgrade paths.
We’re always trying to strike the balance between customizability and stability - Discourse themes and plugins are so powerful because we give them direct access to the underlying browser/framework APIs. That immense power comes with a certain amount of responsibility to keep up-to-date with underlying changes.
Indeed - keeping up-to-date is important. Over the last few months we have invested heavily in our release pipeline to things much better for ESR (previously ‘stable’) users. More details here. You still need to update, but there is much more flexibility around timing & urgency.
As for this specific deprecation, the solution is fully automated. If you can let us know the plugin names, we’d be happy to run the codemod for you and make a PR. We’ve done it for all of the 600+ themes/plugins that we maintain, so it’s a well-oiled machine at this point.