WebKit doesn’t count? Google, BlackBerry, Amazon, Opera, Tizen, etc all maintain different derivations based off of Apple’s work with WebKit, with the most well known being Google Chrome.
(Although I’ll grant that WebKit itself is a fork of KHTML from the KDE project).
WebKit doesn't have nearly the same amount of community engagement as VS Code does probably because it's a piece of infrastructure code rather than user-facing software.
Where is WebKit source and issue management anyway? It's not on GitHub. I don't imagine Apple providing very well for people, who ask for endless options on a product like VS Code.
I suppose the open source Swift language might be comparable, but I don't think it's anywhere in the same league as VS Code regarding the number of users.
I believe WebKit source and issue management is on webkit.org; not being on GitHub seems to make sense to me, as (according to Wikipedia) WebKit was released three years before GitHub was founded. This is the Apple release, not KHTML existing (which was another seven years before that).
(Although I’ll grant that WebKit itself is a fork of KHTML from the KDE project).