It looks like it was already removed, this is just some stuff they missed? Purge remaining ENABLE(SHADOW_DOM) cruft ... Showing 66 changed files with 40 additions and 399 deletions. [1]
I believe most of the shadow DOM stuff was removed almost a year ago after the Blink fork, because the maintainers of the code were leaving with Blink. eg
> Is anyone intending to maintain the feature on trunk? If not, we should simply get rid of code behind this build flag for now; keeping unmaintained code that doesn't even compile isn't healthy for the project of our size.[1]
It looks like this was just some last remaining code behind a leftover build flag.
[1] https://github.com/WebKit/webkit/commit/0e1e0ebf18aaac177366...