https://www.ncbi.nlm.nih.gov/genbank/statistics/ shows the growth of the GenBank database is slowing since WGS (Whole Genome Shotgun) emerged. Is this happening because sequencing centers are submitting their data unannotated (WGS submissions are allowed to be unannotated assemblies)?
It is not really slowing down. The graph that you see in that link is semi-log. So the exponentiality of the growth has reduced. It is not surprising because we have much more data than what we had 10 years back but the rate is not going to increase at the same rate (10 to 1000 is a great change but 1000 to 5000 is not despite the fact the the number of new sequences is greater in the latter). However, new data is being continuously added and the linear growth has not saturated yet.
See these graphs that I replotted from the data in your link.
Nonetheless, the number of new sequences will also slow down in future because we would be approaching the maximum.