The Azure Terraform Provider will be moving from the deprecated ADAL API to the new MSAL API in the unreleased version 3.0.0:

https://github.com/hashicorp/terraform-provider-azurerm/issues/14193

If we do not upgrade, the old provider will continue to use the deprecated ADAL endpoints. Those endpoints will continue to respond according to the deprecation announcement, but they will not received updates. 

Note that the Terraform provider announcement says:

Version 3.0 of the Azure Provider is being built iteratively using Feature Toggles within the v2.x codebase. This means that functionality will be added over time until the 3.0 Release is completed.

So it may be possible to get the functionality we need without the entire v3.0.0 release if it comes down to that.