With the impending support of @property and the existing CSS.registerProperty() support, we should implement interpolation support for custom properties, as tested by https://wpt.fyi/results/css/css-properties-values-api/at-property-animation.html.
<rdar://problem/103341344>
I think we can close this down, the only bug left is bug 249640 and there is no other known work left to complete other than that.