The package fuse3 is available since awhile in sid/buster. Their users however are still using old fuse (v2), e.g. sshfs-fuse. According to this: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=912528 fuse3 is not co-installable with fuse, but a patch is available.
So is there a chance this get fixed and sshfs-fuse and other package can be updated to use fuse3 for buster? Thanks to everyone involved.