Dug through the Webkit lists, found this:
https://lists.webkit.org/pipermail/webkit-dev/2021-February/031709.html
As of last February, someone asked, and got the reply that Apple thought it was a good idea, but apparently it hasn't made it to the planning stage yet.
Update:
Apparently Apple has their own initiative for integration with WASM called SwiftWASM. Release announcements here (and as of the latest release, no SIMD yet):
https://forums.swift.org/tag/webassembly
jzabinskiDolios
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