BeFS has file attributes (so you can say this file is an email I received, add from="Bob Smith <b.smith@example.com>", received="2022-11-15") and it can optionally but by default index all these attributes using an on-disk BTree.
Many modern systems do have file attributes, although they're not always as capable as the ones in BeFS, and none of them to my knowledge chose to add all these BTrees because keeping them up-to-date is expensive.
NTFS provides on-disk indexes, that's where Windows Search stores its data... and how directories work in NTFS. Similar features can be handled in ZFS (the biggest issue is that IIRC its large K/V structure is AVL Tree, so you'd probably need to add a new object at ZPL layer).
What is the big difference is that BeFS exposed the extended attributes easily in the file manager and APIs - compare it with other systems where the APIs were little known or not used - POSIX extended attributes which are very limited (4k), Solaris extended attributes (which let you open a file as a directory) but are Solaris only, WinNT which had the issue of WinNT being a niche among windows systems, etc. etc.
BeOS in comparison required at least considerable porting work, at worst writing from scratch, and that meant the new features were much more visible.
