good questions. i386 was available for org.freedesktop.Platform 18.08 (used by KDE 5.12), not 19.08 (used by KDE 5.14). available arches are x86_64, arm, aarch64. They plan to discontinue ARM for the next platform, so we better get ready for running AARCH64 or to start compiling our own Flatpaks for ARM. Security updates for 19.08 will be done for some time, will stop in 2021 Aug (I think).

Why they drop platforms I don't know. Please feel free to ping them around on IRC (#freedesktop-sdk) or find the issue tracker and complain. I am somewhat tired of doing complaining and asking there [as I did regarding i386 and ARM already; don't remember where was issue tracker]