you may want to explain a bit more what you mean there.
it could make sense to keep the TODO in the .cpp file, no need to get it on every user

Instead of the TODO one could already add an overload of the method now, and deprecate the current one. So that in KF6 times the deprecated could be dumped, and there would just be the preferred variant.

In D24999#555191, @kossebau wrote:

Instead of the TODO one could already add an overload of the method now, and deprecate the current one. So that in KF6 times the deprecated could be dumped, and there would just be the preferred variant.

Nice idea

It's actually an approach to be preferred in such cases, as it moves some of the work to be done at creating KF6 and porting to its new API for users of KF to now and the time span until KF6, keeping the unavoidable hurdle at the actual KF5->KF6 change as small as possible.