Looking forward to this feature landing in PHP 8.6.
I don't often use generic definitions in docblocks today, but when I do, it's a mess. I think this RFC is the right move at the right time, and cost (both runtime and syntax) appears zero to minimal at best. It'd be really great to see generics land in PHP. It's been a long time coming.
I've been writing about this for years. If we ever want generics in PHP, this is the way.