Yes, I think it would be reasonable to drop the overlay storage driver from the whitelist. 
Foremost in my mind was not unduly inconveniencing users on enterprise distros. Docker Inc. appear to have dropped their support for all platforms but the following:
- Debian 9 (and later)
- EL7
- Ubuntu 16.04 (and later)
The kernels in these distributions should support overlay2. I can’t think of a reason why someone would choose to use the older overlay driver when their system meets overlay2 requirements.