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%...

For me it's aperture. I like the interface better than lightroom, and I don't want to pay a monthly fee to have access to my photo library which I only add to once in a while. It's a shame because it's a great piece of software, and even the UI doesn't feel dated, but I just won't be able to run it if I upgrade.

https://github.com/cormiertyshawn895/Retroactive

It also works for iTunes and iPhoto. Sadly it won't fix any of the other known Catalina issues, of course! ;)