In the package rcs
(https://build.opensuse.org/package/show/devel:tools:scm/rcs),
there is a build error for Leap 15.4 and Leap 15.5:
"unresolvable: nothing provides lzip"
Although both distributions provide lzip, and the required
"BuildRequires:  lzip" is given in the spec file.

The issue seems to be, that OBS builds against a
subset of packages (drawing by Jan Engelhardt):
SLE-15.5-Base
`-- SLE-15.5-Update                 <- devel:tools:scm is pointing here
    `-- SLE-15.5-Backports          <- lzip is available from here
        `-- Leap:15.5
            `-- Leap:15.5:Update

Which explains why this can not work. But there are software projects
(such as RCS) which only distribute lzip-compressed release tarballs.

IMHO such a basic, widely used compression tool should be available
in SLE-15.5-Base or SLE-15.5-Update, so that OBS can use it to build
packages. Alternatively there should be a mechanism to provide essential
building blocks to OBS (outside of SLE-.*), so that OBS can do its duty.

At the moment there is no possibility to build software which provides
lzip-tarballs.