Works on slopes?

I'm not an expert but I believe no device takes slopes into account the way I assume you are after. But please tell more, what would you like to achieve? Perhaps experts respond if the background is made clear first.

If it is the field area, for what purpose? If you would like AB-lines that are not straight lines, why? Have you calculated the "error"? It isn't significant anyway for slopes of, say up to 20 degrees or so, or perhaps you feel differently.

I only know the Trimble range, but the EZ-Guide 500 and the FM-750 (when used as light bars only) dont compensate for slope, ie if the tractor is driving across a slope the receiver on the cab top will lean down hill causing a slight error.
The good news is that when you move up to automated steering (ez-steer, Nav 2 controller etc) they are fitted with gyroscopes to quantify and cancel out the error.
Mower offsets are set in the settings, to the nearest cm.
As for measuring the actual field size rather than the plan view size, that I don’t know.

I thought this was not about tilt compensation, tilt compensation just recalculates the guidance line to the field surface. Say the distance between adjacent AB-lines would remain "incorrect" if measured along a sloped field. Field area would match "the bird's eye view".

In order to calculate the true field surface area, one would need to sample the whole field, not only field borders. This would not be practical and would neither match the "official EU definitions of field area", i.e. the figures on official documents and reports.

Even some basic guidance only screens support/include tilt compensation but I doubt any device calculates the field area taking slopes into account. I remember Fendt planned something like this for VarioGuide, early brochures had a button for slope compensation but it was never implemented for real. For field area the error is not significant, some 1.5% for a 20 degree slope. Equally AB-lines would become a mess if the slope would vary along the line. Much better to accept the "error" and ignore the Z-axis variations for guidance lines.

Probably all devices have an option to set implement offsets so that coverage is documented asymmetrically behind the tractor, in front of the tractor and including distance from the antenna etc. etc.

A Topcon x25/x30/35 with an agi-3/4 receiver can be used by enabling "per point data" this produces a file that together with some neat software can produce a 3 dimensional map. A standard feature, really popular in the survey market.
Not quite sure this is the level of accuracy you need though.

Pretty much any precision ag device allows data logging and obviously post-processing can give many kinds of results. I thought we were discussing features of the devices though.

An example from measurements based on Topcon HW (VarioGuide and Auto-Guide 3000).