"Signal" has been used in FRP circles for some time [1,2]. The original FRP stuff was events/signals and behaviours. But I agree that JS didn't use this terminology until more recently. S.js is maybe one of the earlier ones, but that was still over 8 years ago.
[1] https://scholarworks.rit.edu/cgi/viewcontent.cgi?article=651...
[2] https://github.com/14427/signal
[3] https://github.com/adamhaile/S/tree/e897ec1212a073bb1fe695e1...