As noted in T319432#10745119, the deployment hosts are among the remaining install-base for our PHP 7.4 packages from component/php74. This happens via O:deployment_server::kubernetes -> O:deployment_server -> P:mediawiki::deployment::server -> P:mediawiki::common -> P:mediawiki::php, with the predominant historical use case being the ability to run maintenance scripts locally - e.g., as part of scap deployments.
While that functionality has now been containerized (and that use case migrated to 8.1 in T390225), we still have PHP 7.4 packages installed. As part of fully removing 7.4 from production, we should address that.
Rather than attempting to support, e.g., "no present PHP version" in P:mediawiki::php (given the amount of complexity already present in this family of puppet classes), I'd propose that we simply update to 8.1 in-place, as we already have with parsoidtest1001 in T380485. That's arguably the least surprising solution, particularly if we want to (speculatively) retain the ability to run MediaWiki PHP code locally in some contexts.