As of build 12695 (2020-11-30), the EcoCore Package contains builds with .netStandard 2.1 and Core 3.1.
The reason behind the update (besides normal maintenance) was driven by the TurnkeyCore implementation of real-time and network polling reduction work we did for Turnkey(.NetFramework4.x version) by utilizing SignalR. SignalR required core 3.1 and everything else followed from there.