It is already an optional dependency of kio, which is where it belongs. IMO it doesn't make sense to also add it to every application that makes use of it. If there is a severe functionality limitation without it then the right thing to do would be to make it a hard dependency (like dolphin does)

Hmm, Krusader runs fine without kio-extras only with less functionality. So I guess a hard dependency is out of scope here.

I actually thought this is the exact use-case for an optional dependency: kio-extras adds optional functionality to Krusader. And not kio because this is only a library and mainly useless by itself.
And yes, every application that uses kio-extras should have it listed as optional dependency, so that the users know about that.

Don't wanna be offensive, you know the rules here better than me. But right know many users a clueless about this and it would help them if the package would include this information.

But krusader doesn't even use kio-extras, does it? It just uses kio, and kio itself detects kio-extras and offers enhanced kio support to krusader...

And this is already plenty discoverable, both because it's printed at kio install time, and because you can retrieve this info after the fact with:

$ paccheck --recursive --opt-depends --quiet krusader
[...]
kio: unsatisfied optional dependency 'kio-extras: extra protocols support (sftp, fish and more)'
kio: unsatisfied optional dependency 'knetattach: to add new kio-remote entries'
[...]