add a gamerule or feature, such that any entity or mob which is named will have a death message like [name] was killed by [cause]

This might be a bit out of scope for EssentialCommands, but should be easy enough to implement as its own, small mod. Might do this in the future as a rainy-day project. Leaving this issue open until I make a proper to-do list to add it to.

Implemented in my new mod, Server Announce. (It is currently the mod's only feature :P)