Schmitty, dsiders summed it up. FPC/Lazarus is not going to abandon GTK2. But distros are increasingly not pre-installing it. But not as bad as that may seem. Note -
While it may not be pre-installed everywhere, every distro I have tested does have GTK2 available to install from their repos.
Qt5 and Qt6 are, really, really great alternatives. A new GTK2 install needs about 200Meg, Qt5 needs about 50Meg.
Finally, and most importantly, you do not have to choose one or the other. By using LCL, your app can easily switch between widgets. Its important you check, from time to time, if there is a difference and then you will add some {$ifdef XXX} things to manage those small differences. I build GTK2, Qt5 and Qt6 (along with Windows and MacOS) binaries all from the same code.
And, as Zeljko noted, Wayland is far more of a problem than choice of widget sets.
Davo