The vast majority of (visible) scratches come from putting a display in your pocket. Dirt and shit (especially sand. fucking sand) will demolish a screen.
But it is also something where a lot of people are running on decade(s) old information. Even the majority of non-fancy displays are generally at least “tempered glass” and can hold up to most normal wear and tear.
And the Steam Deck has the benefit of (among other things) the sticks being too fragile to just throw it in a bag without the case. So as long as you don’t take the case out at the beach, you are fine and it is protected at almost all times.
That said: I am still a screen protector person. Mostly because it is 10-20 bucks for peace of mind. I have only ever had to replace one once in my entire life but… yeah. But also? My tablet doesn’t have a screen protector (since the vast majority would interfere with stylus responsiveness) and instead relies on a folio cover. And I have zero worries.
I’ll also add that I think the nintendo consoles REALLY fucked people up. I want to say it was the 3ds and everything before it was straight up plastic? And I can’t find anything definitive, but I want to say the switch is also a plastic screen? That is the territory where not trimming your nails regularly enough or picking it up after you had a few crisps can start to cause damage.
The other common argument is it will protect a display in case of a fall. I… mostly call BS on that one. Yes, there are freak occurences where it will land perfectly on its screen on a raised object and cause just enough damage to the screen protector without hurting the display or the device underneath. But a shocking amount of work goes into drop-proofing devices so that your unprotected screen ALSO is very unlikely to crack unless it is falling straight on an exposed metal bolt… at which point your device is likely fucked too. And the extra layer of tempered glass does nothing for a side or corner impact.
Also, the Steam Deck’s analog sticks and the like are far more likely to get damaged than the screen in the case of a “normal” fall.