Up until the release of Catalina, I've always upgraded to the latest version of macOS within a month or two. But some of the changes this time is really stopping me from upgrading.
As of Catalina, there's no sane way to install the Nix package manager without losing functionality because macOS now disallows creating new files in the root directory[1]. Nix stores its packages in the /nix directory and it's not possible to migrate without causing major disruptions for existing NixOS and other Linux users. This is too bad, since apart from Nix being a nice package manager, it also provides a sane binary package for Emacs. The Homebrew core/cask versions only provides a limited feature set[2][3].
[1]: https://github.com/NixOS/nix/issues/2925
[2]: https://github.com/Homebrew/homebrew-core/issues/31510
[3]: https://github.com/caldwell/build-emacs/search?q=support+is%...
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