Not familiar with PWM. I know it's some anti-flicker tech, but nothing else. I've never noticed flicker on Steam Deck's screen [edit: but I'm not very sensitive to it].

Steam Deck does have an adjustable refresh rate screen which can be set to any refresh rate between 40 and 60 Hz. It does not support VRR (real time adjustment), but usually you play around to get best FPS you can, then you lock the refresh rate to the same frequency (unless your FPS is below 30, then you lock it to twice the rate).
However, if you connect the Steam Deck to an external screen that does support VRR, Steam Deck is compatible and will work. Just not with the inbuilt screen.

Steam Deck has VSync on by default to prevent tearing, and it's applied on top, to all games. You can optionally disable VSync by toggling the 'Allow Tearing' option in the quick menu.

Yes it flickers, more noticeable when setting lower frequencies, it personally doesn't bothers me at all, but I never had that problem with other devices neither. I think some people couldn't handle it. Ad of course apart from personal sensitivity there is always the brightness and illumination of the place you are that would affect how visible it is

About the tearing, first of all due the screen being a vertical one any tearing would be vertical. Then if using SteamOS due to using Wayland until recently it was impossible as that forced always VSync. But now there is a new option to allow tearing, I haven't test it. There isn't VRR, well for the device screen, I think VRR worked for monitors supporting it connected to it but not 100% sure.

The Deck uses PWM. I'm one of those unfortunate few that ends up feeling nauseous when using 40Hz. I don't notice the flickering directly, but 10 minutes of gaming or so and my head is spinning. I don't get the same issue at 45Hz or higher.