We did this at stripe when deprecating TLS 1.0, and called it a brown out (I don't know the origin of the term in software).

You do it when you have a bunch of automated integrations with you and you have to break them. The lights arent on at the client: their dev teams are focused on other things, so you have to wake them up to a change that's happening (either by causing their alerting to go off, or their customers to complain because their site is broken)

have also heard this as doing a “scream test” — turn it off, see who screams about it