Even if it is, for whatever reason, not part of the repository you have installed, you can still go on steam’s website and download the package from there… In other words, the worst case scenario is literally the same experience as you would have on windows…
I use and love debian a lot for servers because it is super stable and relatively lightweight, but I definitely would not recommend it for desktops/notebooks, especially not to people new to linux based systems.
It’s super purist about foss, which means you only get free and open source software (no steam, discord, etc) per default and it uses an older kernel (which makes it more stable, but less feature rich and less compatible with new hardware).
If you use something like fedora, linux mint (as far as I heard) or even ubuntu, your experience would probably be a lot better.