How do you label code to be deleted in KF6?

[Glen Ditchfield]

I would like to have some members of KCalendarCore's incidence classes 
removed during the transition from KF5 to KF6, and I'd like to be sure 
that the changes won't be missed in the excitement.

Do you have some preferred way to label such changes?  For example, is 
there some `#if` statement that can be wrapped around the code, such 
that it will be compiled into KF5 but compiled out of KF6?