It is recommend not using custom comparison functions, but instead converting the keys to one of the forms that are suitable for built-in comparators (for instance take look to the Value-to-Key functions). The reasons to not using custom comparators are:
The order of records could not be validated without your code. So mdbx_chk utility will reports "wrong order" errors and the -i option is required to suppress ones.
A records could not be ordered or sorted without your code. So mdbx_load utility should be used with -a option to preserve input data order.
However, the custom comparators feature will never be removed. You have been warned but still can use custom comparators knowing about the issues noted above. In this case you should ignore deprecated warnings or define MDBX_DEPRECATED macro to empty to avoid ones.