That wasn't quite what I meant, and I'm not searching for anything controversial
I usually choose terms that a customer wouldn't use by accident, and in this case, it is 'nighttoys' as I want to search for specific dog toys that can be used in low light.
I don't mind the search term being in the URL, but I don't want to use the term 'nighttoys' in a tag or anywhere else on the details of the page, as it is pretty meaningless in itself. I could probably choose something that isn't meaningless and make it part of the product text(s), but it seems like an overcomplication compared to the perfectly acceptable method used in 4.x.
However, now it seems that I don't need to do anything extra.
When I was fiddling with this yesterday, I added that term to the meta keywords field in my products. At the time, it didn't work and when I searched for it I didn't get any results. Today, it works.
I use Cloudsearch, which updates every eight hours, so I think in between adding the keywords and this morning, the index updated to include my keyword.
I have another one to do today, so I'll see if I get the same result.
Many thanks for your input, Steve.
UPDATE: Resyncing the Cloudsearch index does work. I just tried with a different selection of products, resynced, and searching on my chosen term does display them.