For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:darix:restic/openSUSE_Tumbleweed/home:darix:restic.repo
zypper refresh
zypper install restic
For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:darix:restic/15.6/home:darix:restic.repo
zypper refresh
zypper install restic
For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:darix:restic/15.5/home:darix:restic.repo
zypper refresh
zypper install restic