UML as a methodology seems to have disappeared, but bits of it (e.g. sequence diagrams) seem to have been absorbed by the profession at large.

I think the UML statecharts are a much more useful diagrams than sequence diagrams. It can convey a lot more of the behaviour especially if you use the entire 'Harel' formalisms.

Too bad, they aren't that popular to document behaviour, because sequence diagrams really don't convey that much.

For example timeouts are very neatly described in a harel state chart. How will one describe timeout and timeout handling in sequence diagrams.

Sequence diagrams existed prior to UML. They were extensively used in telecom: “message sequence charts.”

They were part of a telecom language called SDL. There were tools to “compile” to various languages such as C. If you followed their rules, you were supposed to be able to go back and forth between C and SDL.

I only remember this because the biggest tool provider at that time was a French company called Verilog. They preceded the HDL by a few years, I guess.