As far as I know there was Macintosh File System (MFS) which was replaced by HFS/+ and finally APFS.
But ReiserFS was also infamous for its difficult to maintain nature at the time of its inclusion. Linus himself was against a lot of aspects of it. They capitulated to community pressure to include it, but it was always going to be a project with a high bus factor regardless of his crime.
Here it is, the service is called "AFS": https://en.wikipedia.org/wiki/Apple_Filing_Protocol
Unfortunately it was still frequently in use 10 years ago on Apple systems (particularly the time capsule which was only discontinued in 2018) , and there are derivatives in use today. So I’m not sure it’s an apt comparison point.
The Wikipedia article actually links to implementations like netatalk that have updates as recently as a couple months ago. So it is actively maintained beyond what ReiserFS has.
AFS however, does, and I was doing MacOS sysadmin work in 2011 and it was strongly discouraged to use anything other than SMB, which frustrated me because samba3 was a pain and I considered SMB a windows technology at the time.
The time capsule as you mention also discontinued the usage of AFS after the first version (I actually have 3 at home right now still, a 1st gen, 2nd gen and last gen - the tall one).
AFS's use today, not sure, but as mentioned, 13 years ago at least it was strongly discourage by Apple themselves.
I’d bet there’s a significantly larger set of devices active at any given time during the existence of ReiserFS to today that still use AFP.
And I’ll point to https://news.ycombinator.com/item?id=42101313 that says AFP has no kernel support, but something else called Andrew File System does.
AFP, the Apple Filing Protocol, is a network protocol, not a filesystem. It has several userspace implementations. [1]
AFS, the Apple File Service, is AFP plus other auxiliary protocols. [1]
APFS, the Apple File System used in modern MacOS, has read only support in userspace, and there is work on a read/write kernel module that is not mainlined. [2] [4]
HFS, the Hierarchical File System used in older MacOS, has mainline kernel support. [3]
[0] https://docs.kernel.org/filesystems/afs.html
[1] https://en.wikipedia.org/wiki/Apple_Filing_Protocol
[2] https://en.wikipedia.org/wiki/Apple_File_System
[3] https://www.kernel.org/doc/html/v5.8/filesystems/hfs.html
It's a cart and horse problem, nobody wants to use it because they think there's no investment, since developing it could be seen outside as being an indemnification of the person.
Lack of use means that less people even want to invest in developing it further, making it take a back seat, which then leads to even fewer scenarios where it's used, because it's not competitive, and so on.
The Linux kernel rarely removes stuff. This is not something that should be taken as a measure of support; If we want to track reiserFS properly we must follow the commits before prosecution and after, along with some usage trends would paint a real picture.
There's a whole host of dead things in Linux (AFS, which even Apple doesn't support for nearly 2 decades) that isn't removed yet.