For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:khumba:branches:games:tools/openSUSE_Tumbleweed/home:khumba:branches:games:tools.repo
zypper refresh
zypper install steam
For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:khumba:branches:games:tools/15.5/home:khumba:branches:games:tools.repo
zypper refresh
zypper install steam
For 15.4 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:khumba:branches:games:tools/15.4/home:khumba:branches:games:tools.repo
zypper refresh
zypper install steam