The community has built two LSPs, and https://search.nixos.org/options (or "man configuration.nix", if you prefer) shows what NixOS options are available
NixOS and flakes are orthogonal. Flakes are not a replacement for NixOS, they are simply a _lockfile specification_, akin to your requirements.txt or Cargo.toml or such. NixOS is not legacy.
It's more like 7 months of patches. Release (n-1) gets EOL'd 1 month after release (n), and releases are 6 months apart (in May and November). So 23.11 would've been EOL in July 2024.
And since a release happens every 6 months, while you do have an extra month's window, you still have to upgrade... every 6 months.