Currently, GTK3 is still used for the default GUI of some Linux distros, but for how long? Bearing the future in mind, should we develop on GTK3 or GTK4? Or we could ask: How long is GTK3 safe?
Be aware that the Gnome teams throws out the entire toolkit every few years. If you want a more stable API supported over a longer timeframe, use Qt instead.
Web technologies are also a popular alternative.
Depends on what the application is. Web technologies do not make sense for an application like a file editor. Surely the Asker has a reason for asking about GUI toolkits.
Depending on how much you care about GUI designers, bare in mind that Glade was deprecated for GTK4, and you're left with writing XML files by hand or wait until Cambalache ever reaches feature parity.
GTK3 will likely be supported for a long time, since so much software is based on it. Heck, there are still GTK2 apps out there in the repos.
GTK3 will be supported for a long time, but GTK4+libadwaita is a joy to use IMO.