As far as I can tell this was added for the QT port, which was deprecated almost a decade ago.
Pull request: https://github.com/WebKit/WebKit/pull/11162
Committed 261338@main (e5d00e8df212): <https://commits.webkit.org/261338@main> Reviewed commits have been landed. Closing PR #11162 and removing active labels.
<rdar://problem/106369705>