I checked the PlugInIntf.pas for new functions. I relaised, that the PL/SQL Developer 15 is shipped with a file, wich is dated back to 2015, and many new functions does not exists. But, as I compared the two files, it looks, that some new functions are exists in that file, which is not in the previous. For example, ide_getcurrentsytle
Left file is from the PL/SQL Developer14 folder, right side is the 15 version.