It would be helpful if the reviewbot showed a warning when a patch renders an include directive unused in a modified file. Identifying such include directives could be done based on include-what-you-use. Due to the nature of include-what-you-use, this will definitely produce false negatives in the current state. But that wouldn't be a blocker for reducing the number of unnecessary include directives to some degree. However, false negatives would need to be ruled out almost entirely in order not to make this check an annoyance to developers. This will probably require some work in adding annotating include-what-you-use pragmas to the code base, and adding some postprocessing.