Yeah it’s sort of moved from mtb to road for whatever reason.
@The Duckmeister might have some insight into the justification.
Yeah, that B-Link isn't specific to GRX, it's common to all Shadow derailleurs from 9-11-sp, deletable for direct-mounting on most 10-sp. & all 11-sp. models.
The transfer of the Shadow design to road I think is largely due to the trend toward bigger cassettes; having a firm stopper against forward movement of the derailleur body really helps in setting and keeping suitable clearance under the bigger sprockets, where old flappy-pivot designs start mashing teeth and binding - the same advantages it brought to MTB when it appeared around '06-'07. The unwritten (at least officially) extra benefit is that there is enough adjustability to exceed the stated maximum sprocket by some margin (Shimano also have a habit of stating a given derailleur's maximum capacity based on the largest cassette on offer in that particular groupset...).
Direct-mount is good in concept because it removes a potential extra damage point (I've seen several bent B-Links, including one of my own), but its sticking point is that if frame manufacturers want to offer other drivetrain options with normal-mount derailleurs they either need two hangers, or simply don't adopt direct-mount. Most took the latter course, so Shimano quietly removed that capability with 12-sp.