I don't know about the rest of it, but sequence diagrams are alive and well and are basically in pretty much every design doc I write and read.
They are very very useful for letting people quickly gain a shared understanding for what a system is supposed to do (at a high level). They are extremely beneficial IME.
If you are designing a system and need other people to understand key parts of how it all hangs together (e.g. during review or implementation or maintenance) , and you are not using sequence diagrams, I'd urge you to strongly consider doing so.
They are very very useful for letting people quickly gain a shared understanding for what a system is supposed to do (at a high level). They are extremely beneficial IME.
If you are designing a system and need other people to understand key parts of how it all hangs together (e.g. during review or implementation or maintenance) , and you are not using sequence diagrams, I'd urge you to strongly consider doing so.