GDK creates a lot of GDK_SELECTION_* atoms
On my system:
$ xlsatoms | grep GDK_SELECTION | wc -l
14328
That seems like a lot of X11 atoms (for the record, X11 atoms cannot be deleted once created, and remain for the lifetime of the Xserver).
That's probably not new though, it seem it's from commit 4728dd0a