As in "fixed, cannot drift, no windage adjustment". I see a number of designs on the market, already available or incoming, that have this in place. Old Trijicon 1911 integral sight RMR plates, Agency AOS ACRO plate design, conceptually similar ATEI ACRO cut with a shim sight, lesser known Mac Defense plate (according to email I received, sight is bolted down from below). The new Unity Atom 2 seems to have the same thing going, rear sight is a part of the plate.
For some reason such designs bother me. I have my rear irons drifted a bit here and there on my carry guns. I also want a close, if not absolute, vertical alignment of my irons and my dot. I use it to make sure the dot hasn't lost the zero. If I can't drift the irons on a carry gun, I don't want such setup. Am I too much of an OCD these days?