That's what POSIX was for. Keep your scripts and system calls POSIX compliant and you could move from something like AIX to Linux easily.

You will likely have to adapt your scripts for OS-specific or installation-specific tasks like package management and modifying filesystems. In the past I've used Nix (either via `nix run` and `nix shell` or templating in Nixpkgs' `writeScript` or similar) for this stuff to guarantee that I'm always running the same tools regardless of what's installed on the base system. This can free you up to use a different shell, rely on recent features of Bash, use GNUisms in coreutils, sed, grep, find, etc., fix a specific version of jq, use external templating tools, etc. For systemd-based distros, you can even use Nix to manually install system-wide services: just install a package to the default or system profile, and then symlink the included unit files from the profile (not the direct store path) into /etc/systemd. `systemctl daemon-reload` and you can manipulate them in all the usual ways one would with systemd.

Other Unix distros don't have first-class support with Nix so you may need to take some additional care when working out your script (especially the part of it that installs Nix), but if you don't need to set up services this way you can write portable scripts with few limitations that will work across all Linux distros, macOS, probably FreeBSD and maybe NetBSD.

I've never been so lucky as to work at a place that used any Unix flavors other than Linux and macOS, though.

You know what happened when centos retired? Nothing for us. We still use centos 7 at work as we speak.

Depends on where you are in the ecosystem. If you're running your own service, the only flavors that matter and the ones you're using.

If all my machines are FreeBSD 4.11, I don't care if my scripts don't run on Linux or Solaris or SCO or even FreeBSD 4.8 or 14. I might care someday, but not today.

Maintenance scripts need to run on all the versions in the fleet (usually), but setup scripts can often be limited to the latest version, because why not use the latest OS if you're setting up a new machine.

If you're distributing software, yeah you've got to support a lot of variation. If you're at a shop that runs lots of different flavors, you have to support lots of variation. But a lot of people just pick a flavor and update the scripts as needed when the flavor of the day changes.

Trying to keep dependencies and running services as tight and small as possible helps a lot with keeping up to date on security. Don't need to update things that aren't installed, and may not need to update things that are installed but not running (but sometimes you do).