Generational arenas are really nice. I don't know why they are being discovered only now. Like e.g. graphs implemented using generational arenas play nicely with CPU cache.
For anyone who cares, here's my Rust crate that's like petgraph but with generational arenas.
They’re not new this article is from 2013 for example and the general idea is older still. But I think interest in Rust, the ECS pattern and the associated advantage of easier ownership semantics that handles provide explain their wider popularity.