GTK is a 22 year old project.

Both GTK and GNOME have used .99-style release candidate micro numbers, and odd/even minor numbers to represent devel/stable (as Linux once did) for a VERY long time. ~18 years ago, GTK/GNOME decided that major number bumps meant breaking changes.

.99 means "we're absolutely serious about this release candidate" because you know you're going to eat it if you ship a three-digit minor. How embarrassment.

Kids, a bit of community tradition won't hurt you. :-)