@VadimLevin thanks for your question.
As described in #23512 we want to solve the constantly occurring problem of "Unrecognized selector" - to do this we need to move functions out of a category into the Mat class itself.
But these functions rely on code that is in imgcodecs module - so I kept the main bodies of the implementations in the imgcodes module. The reason to change the names from Mat+Converters to MatConverters and from Mat+Quicklook to MatQuicklook is Objective-C file naming convention. A file named like Mat+Converters indicates the file implements a category Converters on the class Mat. Because we are not now implementing a category I changed the name so as not to break the convention. I hope that makes sense

Yep, thanks for explanation!