xorg-server and related packages were moved from main to community in Alpine Linux. This resulted in xorg-related packages getting deleted before the new packages were available (aports#12152).
Until the builders have caught up and built the packages again, you can build them yourself:
$ pmbootstrap pull
$ pmbootstrap aportgen --fork-alpine xorg-server xinit xauth xmodmap xrdb
$ pmbootstrap aportgen --fork-alpine libxfont2 xcb-util-image xcb-util-keysyms xcb-util-renderutil xcb-util-wm
$ pmbootstrap build --arch=aarch64 xorg-server
If cross compiling xorg-server fails in the last step, add !check to the options in temp/xorg-server/APKBUILD of your local pmaports.git clone.
Update: packages were temporarily added to postmarketOS (!1768)