Izaic @Girtablulu is the maintainer of Qt5 and Plasma. There are always compatibility issues with any minor release of Qt5. Many applications rely on private ABI symbols, despite being strongly discouraged by the Qt5 devs, as the tend to change too frequently for most software. We typically wait a few weeks to a month for the dust to settle on the new release and for those applications to get updates related to any compatibility issues. KDE/Plasma fall into this category as well.
@Girtablulu also needs to watch out for these upcoming deprecations which will eventually break a great many applications:
The following modules are part of Qt 5.15 release, but are deprecated and considered for removal in subsequent releases of Qt:
Qt Script
Qt Quick Controls 1
Qt XML Patterns
Having reviewed the entire Qt 5.15 changelog myself: until Plasma 5.21 is released (and stable), there's no real rush to upgrade Qt5. Please be patient.
