I've always liked and used a lot the jpeg conversion under Plugins. Most recent gwenview versions have removed this great tool.
Could someone tell me the most recent Gwenview version that still can use this tool, and I'll revert back to it. And would this version still work ok under Suse leap 15.3

Not sure about the jpeg you're talking about but gwenview no longer does kipi plugins. You could install them though and see if they'll pop up in gwenview.