Terms of Latin origin in plaintext in titles, or some way to italicize them for those who don't recognize them as such?

Having no markdown in titles makes things more simple, and I’d tend towards preserving simplicity here. Formatting is a distraction here, your case is more about using terms that some people are unfamiliar with.

If you don’t know the term, italics might not help much. There are enough cases of users randomly formatting words inside their posts that someone unfamiliar with a term might still consider this a typo even with italics. So I don’t think adding formatting is a true solution to the issue, and would provide only negligible benefits.

Question titles should use language appropriate for the topic of the site. Many of the latin terms you mentioned are useful because they describe very specific concepts succinctly, they have no replacements that are equally specific and still reasonably short. Using these terms is useful, and that’s why they are still popular in certain fields.

Your example is a bit different, I don’t see any actual advantage for the latin term here. It is neither more specific nor much shorter than the plain english replacement. I have to also admit that I was unfamiliar with its use until now, I could of course guess what it means but I can’t remember having seen it before. This is quite different from all other terms you mention in your post, I would not consider these cases equal.

In short, I don’t see much potential for confusion here outside this particular term that isn’t all that useful to begin with. So I don’t think anything needs to change.

One strategy to deal with titles would be to avoid the use of commonly used latin phrases

The phrases themselves are common in scholarly and scientific language. Many of their individual words or derivations are lexicalized in several languages. In some cases they express concepts that don't have a better or clearer equivalent/translation. Some of the phrases are also immediately intelligible to a significant number of readers, by comparison with figures of speech and colloquialisms that to non-native English speakers may seem unintelligible.

I think the hypothetical strategy of avoiding these expressions would be going against long standing practices.

Should we leave the terms of latin origin in plaintext in titles

This would be an interesting feature request (there's probably one around already). There is some sense to allowing italics in titles, but I can't imagine a reason for allowing bold. The intermediate solution of not allowing any highlighting makes some sense in terms of overall consistency.

References:

"Is it acceptable to mix Latin with English?"

"Is it good style to use Latin phrases in English scientific literature?"

The "Markdown in Titles?" question supports no italics in titles (perhaps that should be revisited), so that's what we are stuck with for formatting; but editing must be an improvement, and not damage the post, so rollback incorrect edits.

Variations on an answer (in the wild, not here; but they were asked at Stack Exchange):

1. "Broken tooltip for question link with non-Latin symbols in comments" - The title is what the title is, never break it.

2. "Does Bollywood title their movies using the Latin alphabet or the Devanagari alphabet?" - Translate the title, alienate your viewers, for your own convenience.

3. "What does “Et In Arcadia, Ego” refer to?" - If you don't know, move along; nothing for you here.

As the author you are the ultimate decider, be it question or answer, as long as you are not wrong there is nothing to be corrected; the pedantic should enjoy their own medicine and support being corrected.

For a moderator's error roll once, then bring it up on the site's meta; for a user roll twice, which flags the moderators.

As a practical aside, attempting to incorporate the term in the first sentence (without repeating the title) emphasizes the correctness of the terminology; and makes erroneous changes twice as evident.

From a technical standpoint the title of the question is an URL, and URLs have no provision for italics. If a non-latin character set is to be used in URLs then Internationalized Domain Names must be supported; though that wouldn't help support italics.

To support accessibility a Clean URL is generally preferred. Encoding of non-latin Unicode characters is done using Punycode, and encoding of reserved characters is generally done using percent encoding. While it's technically possible to support many things, even newlines in an URL we need to adhere to the principles of accessibility, search engine optimization, and clean URLs.

It would be easier (much better) to have a local copy of the title that is presented when the webpage is loaded, that would make the MathJax that we have in URLs prettier. Take the example in your question "Ad hoc proof of Minkowski's bound for ℚ(√−19)", the title is converted to ad-hoc-proof-of-minkowskis-bound-for-mathbbq-sqrt-19; if you know to search for mathbbq's you're in luck, otherwise you'll have to rely on the other search terms to land you at that page.

Munging our URLs doesn't seem like a good idea, but having to change the code and database to support local titles for the sake of adding italics is an expensive proposition; in terms of developer time, manually editing some fixups, and expanding the database for the additional string.

This magnitude of change is something that should have been designed in from the start, assuming it would be desired, to allow nice outward facing URLs (easier to translate, politer, better keywords); adding it in hindsight would be a challenge, not particularly difficult but perhaps one that Stack Exchange won't be considering.