For Flarum installations with a large amount of primary and secondary tags, it can become cumbersome to manage each individual tag's permissions, especially when some of those tags may effectively be duplicate in capabilities (not duplicate in the purpose of those tags).
Use Cases:
Here is a screenshot of some of the tags I've already set up permissions for. This is not a complete list, as it is fairly time consuming to set up these permissions and we have a large amount of tags (read: still a WIP).

I suspect this would function by adding a separate section in Permissions, maybe underneath the Groups list, for permission templates. You would define permissions that are identical to the list you already have for tags. You would have a new dropdown item above the "Read" section of the Permissions -> Tags table called "Template", where you could choose:
Justify why this feature belongs in Flarum's core, rather than in a third-party extension
I believe this belongs as a core feature because tags and permissions are a fundamental part of Flarum, it's something third-party extensions integrate into, and therefore the best place to implement it (as a UX and API level) would be core.
Describe alternatives you've considered
I do not know of any alternative mechanisms that would address this.
Just so I understand fully... the core need this would be addressing is to reduce the work of managing the detailed permission when you have lots of tags, right? So this is a matter of scalability in a way.
I am asking because I've repeatedly see that concrete proposals at an early stage inhibit creativity when it comes to UX / features. Starting from the problem to be solved can open up more radical (and better?) alternatives that would otherwise not be considered.
Just so I understand fully... the core need this would be addressing is to reduce the work of managing the detailed permission when you have lots of tags, right? So this is a matter of scalability in a way.
Yes, that is what my proposal would accomplish.
Yes the scalability of the permissions grid sucks. I mocked up a new UI concept to address this a few months ago. The idea is that you configure global permissions, and then rather than re-configuring permissions for each tag like in the grid, you can just add/remove them selectively relative to the global permissions:

Still a WIP as to exactly what the UI would look like to create these rules, but I think this is a step in the right direction.
Please for the new design also consider the consequence of large amount of permission groups, not just tags.
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Most helpful comment
Yes the scalability of the permissions grid sucks. I mocked up a new UI concept to address this a few months ago. The idea is that you configure global permissions, and then rather than re-configuring permissions for each tag like in the grid, you can just add/remove them selectively relative to the global permissions:
Still a WIP as to exactly what the UI would look like to create these rules, but I think this is a step in the right direction.