It is for some airlines but not for BA, where crew can leave their seats when they believe it to be safe. Occasionally the flight crew will ask them to remain longer (if expecting a bumpy climb, or out of SCL where it’s SOP), but this will be clearly briefed beforehand and will trigger an additional manual bing from the flight deck. A fixed 10k rule would be particularly unhelpful on MAN/JER/NCL, where this would eat heftily into available service time.

The 10k bing is typically the seatbelt sign being exterminated. On many airlines this is automatic unless overridden manually, but also serves as a useful reminder to the IFM to turn on the inseat power.