maybe an additional hint for unstable users.
if you are using unstable you may want to keep regularly an eye on the Announcements - NixOS Discourse category.
For nixos-rebuild-rebuild-ng the relevant announcement (including the option flip) was Nixos-rebuild-ng: a nixos-rebuild rewrite
And there is also Breaking changes announcement for unstable which is a thread to keep an eye on. Sadly not all breaking changes are posted there.
Also nixpkgs/doc/release-notes at master · NixOS/nixpkgs · GitHub contains the “running” release notes for nixpkgs things and nixpkgs/nixos/doc/manual/release-notes/rl-2511.section.md at 74e468ca541b70961cd629915f8ea6429942088c · NixOS/nixpkgs · GitHub for nixos things, containing the discussed change here as first item.
I would recomment every unstable user to keep at least a look at the last link (not that as its linking a commit, but the respective nixos changelogs that are edited with the breaking changes) if something breaks unexpected before digging to deep into code or forums.