The Qt Company Publishes A 2020 Roadmap Culminating With The Qt 6.0 Release

[144Hz]

Vistaus Yes. You have to evaluate if you trust Qt Marketing more or less than you trust GNOME and Red Hat’s need for a Stable toolkit.

For now all we know is the GTK 3.24 has seen longer term support than Qt 5.12.

[cynical]

Repeating it again, there's no sign of any LTS at the aforementioned places (in the GTK main page (https://www.gtk.org/) nor in the downloads pages (like https://www.gtk.org/docs/installations/linux/), etc. Where do you see an LTS version? Does it exist? As you say "that requires you to do a bit of work and not rely on marketing speak".

Major versions of Gnome are now considered LTS. You can read more here.