As per this user's comment at mw.org and my own question some days ago, signing looks broken, as in, the related button can never be used, no matter which site or namespace I'm on. My current workaround is adding tildes via the special characters menu, which is meh.
(Triaging as high just in case it is actually a regression that needs to be fixed ASAP, so James can find it at the triage meeting later today).