When trying to search for files in Dolphin, with baloo or whatever indexer kde
uses nowadays, the search fails and an error message about an "invalid
protocol" is displayed in dolphin. The problem is dolphin not finding the
filenamesearch kio slave library, which is packaged in the kio-extras5 package.

I suggest that either dolphin should require kio-extras5 (just recommends
doesn't work if you disable installing recommended packages in zypper), or the
filenamesearch kio should be split to a sub-package that dolphin can require.