Since Mac Catalyst's inception, WebKit has only supported building for it when using the internal macOS SDK. There isn't a conceptual limitation to keep it from building publicly, we just haven't done the work yet. The main incentive here is being able to create a Catalyst EWS queue, so that contributors are aware of Catalyst build breakages before they land.
rdar://57324065
Pull request: https://github.com/WebKit/WebKit/pull/29276