Thanks again for the replies!
I updated my Scrivener to 3.5 and now the learn and unlearn options are back. So that’s one thing resolved.
I started to use Scrivener on Windows many, many moons ago, and was very jealous of everything the Mac version could do that the Windows version couldn’t do. But then I had to re-find special characters, and now I don’t have a personal words list, so maybe there are pros and cons to both.
On Scriv Win, I could tell Scriv to learn a specific spelling and to auto-correct it where necessary. I can’t seem to do that on Scriv Mac. This is a pity as every time there’s a Mac update, I have to hunt for my user or local dictionary, or whatever they’re calling it.
I definitely don’t have LocalDictionary on Mac Sonoma 14.5, on the MacbookAir I’m currently using while on a roadtrip, and I can’t even find Spelling in the Library. When we get home, I’ll have a look at the Mac Mini M2, which uses a Sequoia OS.
If Scriv for Mac came with its own dictionary attached, like the Win one seems to do, I wouldn’t have this problem every time I use a different OS.
For info, the word it remembered was originally added to the Personal Words List on Scriv Win. So I seem to have carried that across. I don’t remember adding it to the laptop, but when I found the Text Replacements option in Keyboard, it was already there. So maybe I did stumble on it before too.
I’d asked it to auto-correct Coilin to Cóilín, an old Irish name, and that still works. This time I wanted it to auto-correct Citroen to Citroën, which now also works.
AND, since updating, I can now ask it to learn and unlearn spellings.
So I’m happy for now, but it would be so useful if Scrivener had the option of its own user specific dictionary, if possible. Aside from the existing language dictionaries it does have, if you get my drift.
Thanks again for all the replies. We’re still on our 3-week roadtrip around the Highlands of Scotland in a campervan and we don’t often get a working signal. So I’m not ignoring any responses! 
Diane