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perl-String-ShellQuote was available in the RHEL 7.4 ppc64le base repo but is not in the 7.5 ppc64le repo. This is a noarch package.
It is still available in 7.5 x86_64 base repo.
As this is not mentioned in the 7.5 release notes (or I failed to find it), this might be an oversight.

Could you be more specific about what product do you use? Server, Workstation, Client? I could not found it in none of them for even for ppc64le RHEL 7.4. For me it seems like the package was never distributed for ppc64le.

Actually, this was first noticed on CentOS, which replicate what's done for RHEL, but I don't have an RHEL subscription to check on the "real thing".

It seems only Server product exists for ppc64le architecture. The architecture was introduced in RHEL 7.2 and there is no perl-String-ShellQuote package in base or optional repository. It's also not in RHEL 7.3, 7.4, or 7.5.

My conclusion is this package was never distributed in RHEL 7 for ppc64le architecture. Maybe it was erroneously provided by CentOS 7.4 and then "fixed" in 7.5.