Good day,
This Ext4 is the one I’m using right now to share stuff between Haiku and the linux external drive, precisely thanks to @PulkoMandy’s advice . So far so good. To be honest, right now I’m doing most stuff on Haiku, so I don’t really “share” stuff among OSes. Windows also supports Ext4 now if you install WSL (windows subsystem for linux) and a linux distro on top of WSL (from the Windows Store). Should bear in mind that at the moment this works only with entire drives, not with partitions (which is my case).
So, if you have a spare drive, you could format it as Ext4 and use it on Haiku, Linux, and Windows. On windows it will be accessible from the File Explorer, as MS docs say.
Regards,
RR