There is a newer and better way to produce appimages.
These appimages use up to date dependencies unlike traditional appimages but can run on old distros. Another feature is that by default they use DwarFS instead of SquashFS because it produces smaller binaries that load significantly faster.
I have created an issue about ensuring icon preview in Dolphin works for these appimages: https://bugs.kde.org/show_bug.cgi?id=519937
I’m not sure where else in KDE support might need to be added or if any metadata besides the icon is read from appimages. Please advise.