Might require that at least one new-style property is present at release time to be able to add more, details are still evolving though.
There is an interesting thread on the Qt ML:
[Development] QProperty and library coding guide
Quoting the most relevant part:
For pre-C++20 (where it’s possible to have zero-size structs), and for compilers that don’t respect the no_unqiue_address attribute, all these struct-instances are put into a union. In that case, a class using QProperty will be larger (by the same amount no matter the number of properties) than the same class in Qt 5. With C+++ 20 and compilers that do respect no_unique_address, the size and layout of these classes will be the same.