What I see or meant as a sign of "dead" is that there will be no new features that Mapbox GL JS has but that are critical for future applications. Like proper or inbuilt vector tiles support.
Unfortunately that whole business unit got canned and the code is now gathering dust somewhere :/ would have loved to open source that one
I'd encourage you to reach out and ask for permission to release it. In the past I've found that the sticking point was usually copyright assignment rather than the release per-se (with a side helping of worry over hidden IP violations), so get them to release a tarball under a permissive license like MIT/BSD, even if your intention is to immediately fork it.
Basically, if you can manage to do the work for them so they can just rubber stamp the release, you can probably get it done.
I thought one of the primary features of Leaflet was its modularity. Would built-in vector tile support be an anti-pattern?
What someone can see as less development activity is simply a sign of a mature product that doesn't need many new features and changes to remain useful — focusing on the core basic mapping needs has always been its goal, and it continues to adhere to it.