Atari used a bunch of different keyboards, most membrane over mylar (made by Mitsumi and others), and some Cherry MX black. As a kid, I used the (godawful) 400 membrane keyboard, the 800, 130XE, 520/1040ST (all mylar), but never saw an MX-based Atari keyboard.
A post I saw today reminded me of the existence of Atari’s early 90s luggable (the STacy) and notebook (ST Book). Never seen either, but was always curious about them. As with the rest of the retrocomputing scene, there are even color LCD and interface mods available for the STacy now.
The ST Book has a somewhat contemporary layout
But the STacy is… odd. In keeping with the productivity goals of Atari at the time they decided to include a numeric keyboard, alas one made for ants. Apple made the (arguably more correct) choice to eschew the numpad in the Macintosh Portable.
Not having used it I can’t be sure, but I’m pretty sure I would’ve rather had full-sized function row and a full-sized arrow cluster above the trackball.