Historical locked intentionally do not show up in tag searches. From the Meta Stack Exchange FAQ:
Additionally, on main (non-meta) sites, historically locked questions are omitted from normal question lists (those on the home page, /questions, and the various per-tag lists), but can still be found by searching for words in the post or title (via either site-search or Google, etc) or by using the search operator locked:1.
... Wait... Uh... crap.
That says "(non-meta)"... so... Uh... Here's the thing. I've known that this doesn't work for a long time but I've never realized that the FAQ specifically says that meta sites aren't supposed to hide Historically locked questions... the point being, I have gone for ~5 years thinking that this was intended... and I'm pretty sure (though I can't be certain) that it's been like this for that long.
Shog added that note to the FAQ back in 2012 and it was rephrased more recently so there's about 9 years during which it could have broken - if it was ever intended to work at all. So, the question now is... how should it work?
I can see the value of using historical locks the way y'all do - in fact, before I started working here, I asked for a way to do that for exactly the reason y'all are using it. Note, point 2 in my question is that the posts don't appear in the active list - which means this bug was confirmed as existing (and thought to be normal!) back in 2016.
So, I marked this status-bydesign when it got cross posted to MSE and linked back here - because (as indicated by the beginning of this post) I thought everything was working as normal - and I'm not actually convinced that this behavior is always wrong. Historical locks are supposed to be an indication of something on the site that shouldn't exist - which is why they're hidden and, while I completely love the way y'all are using them and see the value in not hiding them, I'd much rather give y'all a better way of doing what you're doing that supports the community project rather than using a bad lock because it does what you want it to do.
At this point, I can change the status back to status-review and actually see if we can fix how historical locks work on meta sites... or I can add it to my MSE question from 2016 and see if we can actually build a better tool. Thoughts? I suppose "both" is also an option.