I still use Java 8 even on personal projects. I'm migrating those to Kotlin, so I'm at least not held back with regard to programming language features. I could upgrade those to Java 11 (or 12), just haven't prioritized taking the time to do that over other tasks, and the benefit would be minimal for these particular projects.
When using Java 8 language, I'm just happy to have lambdas and streams. Java 8 was the biggest improvement to Java language since 5 -- and I'd say it's a bigger leap that 5 was.
Generics, in terms of scope, is a bigger feature, for sure. And generics make lambdas more usable. But the distance between generics and their alternative - manual casts throughout - is larger than the distance between lambdas and anonymous inner classes.
Speaking of dark matter coding, story time: a spare time project of mine involves bringing an actual, pre-generics codebase into the not quite as distant past. That beast still has real, living users! Surprisingly, a frequent complaint is running out of heap memory, so they are happily sharing their magic -Xmx incantations with values that would have been outrageously high when the code was written.