We have a manual for each release, and given I wrote my last post in this thread in April 2021, the stable manual was then likely for 21.04 given the stable in the URL.
Using the current stable manual (link currently being for our Lubuntu 22.10 manual) link, the link is
https://manual.lubuntu.me/stable/3/3.1/3.1.8/sddm_configuration.html
ie. the 3.1.9 section is now 3.1.8
If you have issues finding a page due to age (we have a new release every six months in April & October, thus it’s pretty easy to guess if you’ll have issues) just use the SEARCH function in the left panel of the manual; I just did a search for “sddm config” and had the new page URL.
As a reminder as to what manuals we have available in published form, this page can be useful - Manual links currently available 22.04 (lts) & 23.04 (stable)