I checked with EcoMobile.
The chat rep suggests that thier VZW/CDMA plans will be impacted by the CDMA retirement eventually, but the impact is still unknown. All they know for sure is that they are no longer be able to activate 3G/CDMA only phones.
They are accepting 4G LTE phones and can activate them over LTE. Service should continue to work with VZW as long as EcoMobile has a relationship with them. They already offer 4G LTE data on thier "CDMA" plan, they may need to update thier terms. I suppose if the relationship breaks down after CDMA goes away I can always try another 3rd party that has a relationship over LTE with VZW.
I think I have what I need to confidently move forward now. Atleast I know the Samsung S8 version G950U is the correct one to choose to be compatible with VZW.