Support i386 extensions in 5.15
Freedesktop 20.08 runtime and sdk have the extensions: org.freedesktop.Platform.Compat.i386, org.freedesktop.Sdk.Compat.i386, and org.freedesktop.Sdk.Extension.toolchain-i386. As KDE runtime and sdk are based on Freedesktop and reusing Freedesktop extensions where appropriate, I suggest turning on support for i386 compatibility, you already have the /usr/lib/{ld-linux.so.2,i386-linux-gnu} symlinks in place, so unless I'm missing something, it add zero maintenance requirements and nothing is actually being compiled or packaged in the KDE runtime and sdk.
This is probably the major blocker for having Lutris moved from Flathub-beta to the main Flathub repo due to Qt5 dependent Lutris runners and the i386 requirement. edit: One of the Lutris package maintainers gave a negative answer about KDE runtime switch but it still makes sense to have i386 support for possible future users.
Except for Lutris, I don't know any other potential use case for this, maybe if Davinci Resolve will be packaged (lib32-libpng 1.2 dependent) but this is an OpenCL mischief that's not working even with Mesa's OpenCL.
Note that I'm not a contributor to the Lutris package so I'm linking here the most relevant Lutris package bug report to further discuss this with the package maintainers if needed.