It is still possible to provision the Panasonic KX-TGP500B01 on 3CX version 16 (tested with 16 update 3), even though it is deprecated, by following the instructions outlined here.
The phone template is no longer available but can be obtained by fetching it from 3CX version 14 or 15. Most people no longer have this running so as an alternative, you can grab the template from this article. The firmware files are still available for download from the 3CX website (see forementioned link).
Note that the same limitations to the phone usage apply, and with the template it is not possible to add the base station as a FXS/DECT device. Only the first handset is provisionable, additional handsets must be configured manually.
The template also works to provision a KX-TGP600CEB base station.