I meant old code where you have the source, and can recompile it for either i386 or x32, but trying to compile and run it on 64-bit platforms doesn’t work because the code makes assumptions about pointer size. In that case building for x32 seems a slightly cleaner option, and the extra performance might be useful, depending on the application.

If you have only an old binary then yes, an i386 architecture Flatpak would be the way.