Derailleur-shifters compatibility Shimano GRX Shimano writes in the description of the Shimano GRX Shadow Plus RD-RX812 11-speed rear derailleur that it is compatible with shifters GRX ST-RX810 and ST-RX600 shift/brake levers.
Should I understand that it is &quot;only&quot; with these shifters because of a pulling ratio different from their road but also from their MTB standard? An information that I can't find anywhere. Or is the derailleur a standard road one compatible with their road shifters?
The rear pull ratios used by Shimano are:

SIS v1 for Dura-Ace 7400 only, which was about 1mm cable to 1.9mm derailleur movement
SIS v2 for everything excluding that, up to the introduction of 10-speed MTB (i.e. including 10-speed road, which came before 10sp MTB), which was about 1:1.7
Dynasys / 'New MTB' - which was introduced for 10-speed MTB and kept for 11 &amp; 12-speed, which is about 1:1.2
'New Road', which was introduced for Shimano 11-speed road, and subsequently used for all 10-speed road (which includes gravel) products introduced since the introduction of that, which is about 1:1.4

Essentially with ever more closely spaced sprockets, it emerged that while pulling less cable means a shift requires more force, it makes shifting more accurate/precise/easier to setup. Hence after the introduction of 10-speed MTB which pulls more cable per shift, and the introduction of hidden cabling for second-generation 10-speed road, which adds more right-angles/friction, Shimano came up with an intermediate shift ratio between MTB and the previous standard. Since the old ratio didn't work that well with hidden cabling &amp; 10-speeds or more, they essentially have one standard that is used for all new 10 &amp; 11-speed, and probably will be kept for 12-speeds as well.
So the RX812 is not '11-speed', since the RD is not indexed. It will work with shifters that use Shimano's new road pull ratio, which is anything GRX (10 or 11-speed), anything 11-speed, and Tiagra 4700 10-speed.
There are some older 10-speed road shifters using the standard-for-everything-for-a-quarter-of-a-century-pull ratio, but they are no longer in the Shimano range.
The thing specifically with RD-RX812 is it's marked for a 1x drivetrain, and Shimano don't have 1x levers for road (only for gravel), so they only show it compatible with the GRX levers, since these have brake-only levers. There's however nothing stopping you using it with, say, SL-4700-R, which is a flat bar 10-speed lever, and a Deore 10-speed 11-42 cassette.