I didn't say it didn't "let" you. I said it's not supported, as in, the Linux vendors don't consider that to be a feature of the OS that you should actually use.
I've been a Linux user for 25 years. You can reconfigure the OS to use additional repositories. It may or may not work, and only if there is a repository specific to both your distro and its version. But it's not a good idea.
In particular, OS upgrades are very likely to break. Being able to upgrade itself is a basic requirement of any modern OS. If your Linux distro corrupts itself on upgrade or fails to do so and you file a bug report you'll be told to remove any third party software because that's not supported.
This would be like if your Mac started crashing on boot because you downloaded a word processor from a website, and then Apple say "sorry, we only support apps coming from the app store". They don't do that, but Red Hat or Canonical will.
I've been a Linux user for 25 years. You can reconfigure the OS to use additional repositories. It may or may not work, and only if there is a repository specific to both your distro and its version. But it's not a good idea.
In particular, OS upgrades are very likely to break. Being able to upgrade itself is a basic requirement of any modern OS. If your Linux distro corrupts itself on upgrade or fails to do so and you file a bug report you'll be told to remove any third party software because that's not supported.
This would be like if your Mac started crashing on boot because you downloaded a word processor from a website, and then Apple say "sorry, we only support apps coming from the app store". They don't do that, but Red Hat or Canonical will.