Turns out it was removed temporarily because of internal architecture changes, and never reintroduced because of lack of adoption. Of course the biggest barrier of adoption is Safari not supporting them, so talk about a self fulfilling prophecy.
> This feature was originally removed temporarily during multiprocess bring-up, and actual usage on the web has been pretty low. We're willing to reconsider if there is significant demand.
IIRC, it was only even available through WebKit1 (i.e., the single-process WebKit API) and never through WebKit2, so it will have gone from Safari when Safari moved to WebKit2, even though the implementation lived on in WebKit for longer.
It was only 4+ years after it was dropped from Safari that anyone started to ask about Safari support for it again, so it had kinda fallen to the wayside due to lack of interest.
Here it is in the standard: https://html.spec.whatwg.org/multipage/workers.html#dom-shar...
Safari/Webkit removed it in 2015 (edit: maybe even earlier!) and never reintroduced it: https://caniuse.com/sharedworkers
Here is the apparent reason for cherry picking features:
> The implementation of Shared Web Workers was imposing undesirable constraints on the engine. It never gained any adoption.
https://stackoverflow.com/a/48193804
Edit: Here is the ticket for it’s reinclusion https://bugs.webkit.org/show_bug.cgi?id=149850
Turns out it was removed temporarily because of internal architecture changes, and never reintroduced because of lack of adoption. Of course the biggest barrier of adoption is Safari not supporting them, so talk about a self fulfilling prophecy.
> This feature was originally removed temporarily during multiprocess bring-up, and actual usage on the web has been pretty low. We're willing to reconsider if there is significant demand.