I had set a PIN on this SuperSU version, which was systemless, and have now forgotten it. As it is systemless, the pin code fle should be in /su/etc, though I cannot access this through recovery (TWRP), I can only see it on the booted Android, where I of course can not remove it.
Is there a way to get to that pin file in Recovery?
I have found that I could mount the /data/su.img file within Recovery. I had done so using
mount -o rw /data/su.img /mnt/temp1
and removed the pin file from there. Sorry to disturb anyone who was notified about the issue.