Hi,
If the plugin uses only the APIs that didn't change between 2013 to 2019 (and very few changed), the same assembly might work with AutoCAD 2019.
The only real break of compatibility with AutoCAD .NET occured with the 2013 version and the split of acad.exe into acad.exe and accore.dll.
The simplest way to know if there's a compatibility issue with your plugin is to to load it in AutoCAD 2019 and test if it works as expected.