I would have guessed that Ubuntu would install it by default since its a very common way to get stuff from the internet (when in the terminal), but apparently not (the other option is wget which is most likely installed, but that uses a different way to get the stuff).

You should be able to install curl with sudo apt install curl

That should be easily solved with: sudo apt install curl

curl is a good tool to have in general, you can install it with sudo apt install curl

Wow, interesting. You may be able to install curl to fix that like this:

sudo apt-get update
sudo apt-get install curl

Can't hurt to try

sudo apt install curl

sudo apt install curl

sudo apt install curl

Hmm… ProtonVPN team solved this in better way. They put the repo configuration stuff into DEB file, so it’s just a matter of double clicking it and clicking install

I was wondering how they'd solve signature checking and key installation - and looking at their page they seem to recommend skipping checking package signatures which, to be honest, isn't a super good practice - especially if you're installing privacy software.

Please don’t try to check the GPG signature of this release package (dpkg-sig –verify). Our internal release process is split into several part and the release package is signed with a GPG key, and the repo is signed with another GPG key. So the keys don’t match.

I get it's more userfriendly - and they provide checksums, so not a huge deal, especially since these are not official Debian packages, but the package signing has been around since 2000, so it's pretty well established procedure at this point.

Is curl so untrusted that you would prefer to use 3 commands (one which still needs root permissions) instead?

Agreed it's not best practice. But when somebody is saying the individual step-by-step is too complicated, you want to give them the simplest command possible. Even if it is more risky. It's a trade off of accessibility versus complexity