Right, but there are different monitor sizes out there (1366x768, 1920x1080, 2560x1440, 3840x2160) so that's still a fingerprinting vector. In addition, because of various user settings (eg. whether they're using compact theme or not, various OS settings), "full screen" on a 1920x1080 monitor for one person might result in a different viewport size for another. That also becomes another fingerprinting vector.

It's mainly a problem for macOS users, since we tend to adjust the application dock to quite individual preference. Windows users will with full-size window mostly end up in the same few groups. Linux users to a lesser degree.

I use tree style tabs and though my browser is full screened, I sometimes get popups from people and websites trying to be clever, thinking that the only reason a window seems full screen but isn't must obviously be those darned dev tools.

I quickly close websites like that (and blacklist them in my pihole if they're particularly offensive in their messaging) but for people like me these protections do actually help.

I'm not sure if that's still the case, but I definitely remember Tor Browser throwing a warning you when you try maximising the window for this reason.

Probably but there is a lot of space even on a 15" laptop screen. I usually have my browser windows half width. The other half is an editor, terminal, whatever. I'm full screen only for sites that actually use all the space. AWS and Google console, vSphere, spreadsheets, etc.