Here's a maybe-workable way to get this operational for text diffs -- we can use unicode combining marks to "style" the text:

The dog said "m̶e̶o̶w̶ w̳o̳o̳f̳". The d̶u̶c̶k̶ c̳o̳w̳ said "moo".

At least on my system, that looks like an illegible mess, though. We could fiddle with different characters but I suspect this isn't too promising as an avenue.

Here's what I see, seems pretty much impossible to read:

These are more readable but it's impossible to tell which is "-" and which is "+":

• The dog said m͓e͓o͓w͓ w̟o̟o̟f̟.

These are a little better-ish? Maybe?

• The dog said m̌ěǒw̌ w̭o̭o̭f̭.

I also suspect most users opting out of HTML email are using some 1970-era terminal client that they compiled from C by hand and won't have new-fangled unicode.

I pushed for some feedback downstream: https://phabricator.wikimedia.org/T75851#2359538

It also might not be terrible to just dump a block of links into plain text email:

EDIT DETAILS
Description: https://dev.org.com/transaction/detail/xyz/
Ancient Lore: https://dev.org.com/transaction/detail/abc/

(Usually there's only one, but if you add 5 custom text fields it seems like putting five separate sections into the mail is a bit silly. Although maybe that's fine, too.)

I could also make the URIs very slightly prettier if we go down this route.