Won't this throw error for current usages like https://github.com/cakephp/cakephp/blob/d70712c983e6879bfcddd67d975d4b8ef3328aae/src/Database/TypeFactory.php#L33 ?

This goes into 6.x right now, we can backport it, but thats indeed a bit breaking.
Maybe for 5.next and a new minor?

technically nothing can break, its just a code style change. I'd say this can stay in 6.x

cakephp/cakephp#19391

My point was this rule is a bit too pedantic for my liking.

If the latest PER standard allows such partial usages then I would be in favor of keeping it.

Partials make the review much harder for humans IMO, as you need to scope in the context then always (you cannot directly see if this is root level or nested namespace).
For machines there is no difference of course.
But handling this manually is a bit more cumbersome.

Disallow partial uses

Pros

• One canonical form: every external class is in the use block. Easy to scan dependencies.
• Refactors and static analysis are more reliable.
• Enforces aliasing for real collisions, which is explicit rather than implicit.
• Matches the spirit of PSR-12 / most modern PHP style guides.

Cons

• Forces use ... as Alias for name clashes, which some find uglier than a short partial path.
• Loses the "namespace-as-context" readability in domain-heavy code.
• Migration cost when tightening the rule on an existing codebase.

I don't mind either way, i just thought its obvious to keep it more consistent.