Looking through the gitlab, it seems the backport of this hold gesture to GTK3 was rejected for good reason. Seems very unfair to imply it was done out of sheer spite.
It would break a lot, require a new API, and devs reworking a lot of programs.
It's also completely reasonable just from the POV of not accepting major new features in GTK3 when GTK4 exists.
Devs likely expect GTK3 to be feature-stable, given GTK4 has been out a while and GTK5 work starting soon. It's at the tail-end of its life.
If somebody wanted a major new feature in Python, for example, I wouldn't be surprised if the Python team gave it the go-ahead for Python 3 but not Python 2. GTK3 is done, they're only really doing bug fixes now.
Nobody expects new features to be added to Plasma 5 or Gnome 45.
It's 100% the right decision not to keep adding features to an old widget toolkit that has been superceded by GTK4 and is almost EoL.
That issue aside... good. Seems like a nice feature.