Hi @SLG,
I’m sorry to hear of this development. Processing Python mode has no longer an active maintainer, unfortunately. This is a sad state of affairs, and I wouldn’t count on the Add Mode… / Contributions Manager way of installing it to be fixed soon. In the past I managed to copy manually the folder with the mode files to make it work with Processing 3.5.4, which can be a bit confusing for students.
My personal choice has been to abandon using Processing Python mode and to start using Thonny IDE + py5 and the thonny-py5mode plugin. There can be some installation issues on Macs and Windows, but at least there is an active community around the project.