I see that, once again, you know how to keep an eye about the problems, Peterweb.
That written, yes, with third-part programs, they don't "defragment" SSDs in the classical meaning of the term, meaning that a third-part application provided for "defragmentation" usually cannot work on SSD - or dedicated tools operated otherwise, (and I guess optimisation is effectively a more dedicated term). However, these are rather words to say different physical implementation of a solution to a same given problem, (at this near that the operation is unadvised on SSDs, due to the numerous operation making the SSDs cells rapider definitively used, as you surely know, and more about high-memory cells than about elementary cells). We also can suspect that kind of defragmentation/optimisation has proportionally less gain for any lower latency on SSDs, making the usage even more useless, (as a separate old-school Data partition, as I consider, which then brings more problems than any notable gain).
Anyway, the so-called defragmentation is operated by Windows in a transparent manner, and, as far as I remember, Norton does also the optimisation regularly. I know it, because it occurs without demand/invitation, and it slowers my Windows Updates, coming back rapider once I have the Norton notification meaning the optimisation is complete.
That written, about the Norton side, I effectively don't remember to have formally noticed the defragmentation part, but since I have my SSD MacBook (an integrated memory, in fact, hence even slightly rapider, but the base memory is the same as in SSDs) for 7 years now, I'm more than certain to have noticed the defragmentation on Norton side when the windows opened, whatever on the MacOS partition, or in the Windows in Bootcamp. Only the last time, when I noticed only things like temporary files deletion. This last time, I have effectively not noticed the defragmentation denomination.
I can choose to deny the optimisation of performances by Norton, in MacOS, but not in Windows. I'm trying to avoid it the most possible, and to manage this feature trough the OS and manually when needed, but it seems that the Norton feature auto-launches in the Windows version of Norton.