It is extremely important that tablet settings should default to preserving 1:1 aspect ratio ("letterboxing"). They currently default to deforming the coordinate system.
Affected version
Debian, GNOME Shell 3.38.5, X11, although I can confirm that the issue still seems to be present: https://gitlab.gnome.org/GNOME/gsettings-desktop-schemas/-/blob/master/schemas/org.gnome.desktop.peripherals.gschema.xml.in#L161
Bug summary
Tablet settings' "display mapping" currently defaults to transforming the tablet inputs to stretch over the screen dimensions. No artist would ever want to use a tablet in this mode. Having the tablet default to that mode can cause them somewhat serious setbacks when the screen ratio is close to the tablet ratio, it will mess them up but it wont be obvious to them why, and they may just blame themselves. The setting should default a 1:1 aspect ratio. This setting is available, but for some reason, isn't the default.
What happened
Currently, in the settings for wacom tablets, under tablet settings, there is a setting that determines how the position of the tablet pen maps to the position on the screen. The default setting transforms the tablet inputs to fit the screen, squashing the X or Y axis to fit. There's another option (and this is the correct option) to maintain the aspect ratio of the tablet, but to have some of the tablet letterboxed off from accessing the screen. This should be the default. The impacts on the user of this not being the default can actually be pretty awful. I'll tell you what happened in my case.
Most people who draw learned on paper. When you draw in reality the aspect ratio is always 1:1. If you have a different aspect ratio on a tablet, you wont be able to draw properly. Every line you draw will have the wrong angle. When you try to draw a circle, you'll get an ellipse. If the aspect ratio of the screen and the tablet are close, they might never understand what's going on. You might say, "well maybe drawing circles is harder than I thought. Maybe the resistance of the friction is different than it is on paper and I need to get used to it". But it would harm your development as an artist to get used to drawing with deformed coordinates, and when you switch to another computer with a different screen aspect ratio ratio, you'll have to repeat the whole learning process again. It makes no sense at all as a setting and no artist would knowingly use it, unfortunately, I think a lot will probably end up using it unknowingly.
I can't imagine how much damage this would do to someone who is learning to draw for the first time on a tablet with the wrong aspect ratio.
One of the reasons they wont suspect that the problem is the tablet aspect ratio is that they wont be able to imagine why a desktop would would ever allow a tablet to be used with a skewed aspect ratio. No artist would ever want to use a tablet in that mode. There is no use for it.
For me, the dimensions of the tablet and the dimensions of the screen were close enough that I kinda had no way of knowing that this was the reason I was having so much difficulty acclimating to the tablet. It didn't occur to me to check until a friend mentioned their linux installation having this terrible default in a much more conspicuous way, on their double-wide multimonitor setup.
What did you expect to happen
If a user draws a perfect circle on the tablet it should come through as a perfect circle on the screen, not an ellipse.
If switching to the 1:1 ratio requires the user to do it manually, for some reason I can't imagine, then simply refusing to support the tablet until the user manually calibrates would still be preferable to the skewed default (at least, it must in the cases where the screen and the tablet dimensions are close but different, which is when the confusion occurs.). I don't mean to suggest that this is the solution, but it's meaningful that the current behavior is worse than that.