This didn't work good enough on Windows. Haven't tested it yet on Linux but I guess it would be the same. Colors were messed up if I export the layer for flattening to PDF 1.4 and then convert it to PDF 1.3 with Ghostscript and import it back to Scribus. It doesn't work that way.
What works though is to export the whole thing to PDF 1.4 then to convert it to PDF 1.3 with GhostScript. But in that case some lines of text on path got messed up. Not in PDF 1.4 which I made with Scribus but in PDF 1.3 made by Ghostscript.
Another way of doing it is to export the artwork to TIFF, from Scribus then to print that TIFF to PDF 1.4 with PDFcreator (unfortunately it works only on Windows it seems) then with Ghostscript to convert that PDF 1.4 to PDF 1.3. That all works good, but there is one problem: text is not 100% K. And it's not overprint or anything like that. The file is just flat, raster image.
When I try to import the TIFF (before I print it to PDF) back to Scribus, it doesn't work. No idea why. Same thing with PDF which got from the TIFF...
Lot's or issues, unfortunately.
I don't know why the RIP for plates insist on PDF1.3 but that's how it is.
Otherwise Scribus got some great features which I didn't even see in paid software! Yeah the way things work in Scribus is a little weird and uncommon let's say, but it works. Oh, the undo is not working on Windows and that's crazy

)
Maybe I have to look into option of getting Acrobat Pro and to try to print the file from Scribus to PDF with Adobe printer and see what happens. That solution would be for Windows only I guess.