Updated successfully OTA, but with a slight hiccup, to LineageOS version 18.1-20211025-NIGHTLY-pro1, security patchlevel October 1, with GApps and Magisk kept intact.
The hiccup was that the phone, after restarting and then displaying the LineageOS animation for 20 minutes or so, booted into some kind of recovery, showing this screen (but after hitting 'try again', the system came up without problems in normal time, i.e. a few seconds) :
Now of course I wonder whether this was caused by something in the update or maybe my phone is developing a problem...?