The only practically relevant Linux environments you can't expect these things to exist are stripped down distros used for docker images, like Alpine (where you don't expect much from the system anyways) and Android (not sure about the latter, haven't used it in production in quite a while, but afair it's as non-standard as it gets).
Ubuntu has had a non-Bash system shell (dash is /bin/sh) since 6.10.
> no systemd
Without getting into objected-on-principle distributions like Devuan, tons of stripped-down and embedded linux distros besides Android and Alpine don't have systemd, e.g. DD-WRT, OpenWRT, TinyCore. Heck, launchd is more similar to systemd than SysV init was.
> /dev or /proc or /etc
Other than the parts specified by POSIX, the /dev filesystem is the wild west across tons of Linux and Unix systems, and expecting further standardization in behavior from it is a fool's errand (e.g. device name selection, device ID assignment order at boot, presence of metadata-symlinked directories like by-id, overlay/loopback devices ... all of these are highly variable even across modern, systemd-using Linux distros). I do miss /proc, though I don't miss the file descriptors it costs. /etc and XDG standards for configs are nice conventions to be sure, but a significant minority of Linux software breaks with those conventions (all-/opt install locations, anyone?).
> different filesystem,
Sure, APFS isn't ext3/4. Neither are XFS, ZFS, BTRFS, and so on. ext3/4 is a tenuous standard at best, and many businesses make a point of preferring other filesystems on Linux.
...like, I think you might have just been getting lucky and using a fairly similar set of Linux distros such that MacOS was a big switch. There are plenty of valid beefs with MacOS's divergences (don't get me started on an OS that claims certified POSIX compliance while providing a plethora of low-level system APIs documented to break in the presence of fork(2), or the not-quite-superset clusterfuck that is FSEvents vs. MacOS's not-quite-BSD-complete kqueue implementation, or the Sophie's choice of bizarre xattrs behavior vs. the historical accident that is aliases), but what you listed isn't even remotely standard Linux behavior, much less Unix behavior.
