Just to report, in case it affects anyone else, I contacted Steinberg Support on 28 February 2021 asking how I could log out of the forum (as I habitually log out of sites). I received a reply yesterday, 2 months later.
I hadn’t thought it a complicated question but not only they didn’t answer. Instead, they logged me out of the forum, which I hadn’t asked for! I’m still none the wiser about how I sign out from here.
So if you turn to them for support, be prepared for a long wait and an inappropriate response.