Sounds dangerous. Later commits on master might actually add more if(feature-flag) statements, so if you then just mindlessly merge the cleanup branch, you'll miss the added ifs.

I'd prefer to create a cleanup branch like any other feature branch only when actually going to clean up, and spend the extra cost getting back in context, studying all the if(feature-flags) from master. Otherwise you might miss some, or you might forget some interaction that you learned after feature deploy.