This concerns all packages still using a workaround like -DACCEPT_USE_OF_DEPRECATED_PROJ_API_H=1 instead of properly having ported away from it.
"PROJ 7 will be last major release version that includes the proj_api.h header. The functionality in proj_api.h is deprecated and only supported in maintenance mode. It is inferior to the functionality provided by functions in the proj.h header and all projects still relying on proj_api.h are encouraged to migrate to the new API in proj.h. See Version 4 to 6 API Migration. for more info on how to migrate from the old to the new API."
See also: PROJ 7.0.0 release notes https://proj.org/news.html#id58
Migration guide see $URL.