@Samwilson I think that if an extension maintainer uses tags, that in itself would be insufficient.
Of course, one is free to do whatever, but I don't see the benefit of standardising an endless number of differing policies. That's counter to the benefit of standardising. The cost of more policies goes beyond this page. It also affects actual site-admins in how likely it is they will understand, remember, and successfully execute each MW upgrade. Having more strategies definitely makes this harder.
It's fine, of course, for an extension repo to have tags, but for maximum impact on mediawiki.org and ease of upgrading for its users, it should also follow a standard policy ("master" or "rel").
For example, they could follow policy "rel" by updating the REL branches after each tag release to be in sync with the latest tag supported for that MediaWiki REL.
Or, even easier, follow policy "master", by using a "develop" branch for the unstable state, and periodically fast-forward "master" to the latest release tag.
In addition to causing confusion for site-admins, it would also ensure the ExtensionDistributor can still be used, given it only offers "master" and "rel". If an extension maintainer insists on using unstable master, tags only and no rel branches; their users would need to download the extension by other means - in which case the Compatibility page matters less.