It seems that it is unused and can't find any reference besides below: https://github.com/WebKit/WebKit/blob/60f1940c69fed978c05c98fd4a2b522bd8c59f3a/Source/WebCore/svg/SVGElement.h#L50 Just have to remove this line. Thanks!
Committed 270448@main (e043a48cf24e): <https://commits.webkit.org/270448@main> Reviewed commits have been landed. Closing PR #20222 and removing active labels.
<rdar://problem/118178917>