For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/filesystems/openSUSE_Tumbleweed/filesystems.repo
zypper refresh
zypper install reiserfs
For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/filesystems/15.5/filesystems.repo
zypper refresh
zypper install reiserfs
For 15.4 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/filesystems/15.4/filesystems.repo
zypper refresh
zypper install reiserfs