Low nvram notification is nothing new, and is becoming increasingly common for that old model as it's running out of nvram space with every new firmware adding new nvram settings. It's part of why Asus is leaving it on the 386 branch most likely, as 388 probably adds even more nvram settings.
This 9 years old model is starting to show its age, with Asus having added lots of new features to it since its initial launch in 2013.
I might have to start considering removing some things from the RT-AC68U to save some nvram space. Reducing the number of OpenVPN clients would be such a thing, as each client consumes around 1 KB of nvram even while not in use.