[Development] Deprecation/removal model going into Qt 6

[Kevin Kofler]

Lisandro Damián Nicanor Pérez Meyer wrote:
> Boost is **exactly** the best example of why Qt should not go the same
> route. It's totally problematic to have more than two stacks around,
> and we **will** have Qt5 and Qt6 for a long time.

In fact, Fedora even still ships Qt 3 and Qt 4 and they are not going away 
any time soon! Some applications will never get ported. What's the benefit 
of the new features in new Qt releases if your application works just fine 
without those features? The most important feature in a library is backwards 
compatibility!

        Kevin Kofler