Last week I purchased a new (used) iMac with Catalina. Installed Scribus 1.5.5. It runs smooth and flawlessly. Though I don't like the GUI, I think I will get used to the new GUI layout soon.
One thing I noticed: Setting up the preferences doesn't seem to work: When I start Scribus for a new document the settings differ from the ones defined in »Preferences«.
With the 1.4.8 version on El Capitan I could open an existing document by double clicking it. This feature is gone: When I double-click a Scribus file in finder, Scribus opens with the Document Setup Menu, but not with the selected file. However, when I right click a file in Finder and select »Open with« Scribus, the document opens fine.
I don't know whether this is a minor glitch of macOS or Scribus. Nothing I worry about at this moment because I can use it and it seems to be stable enough for production purposes.
Thanks to all the developers who made this possible!