The providers are what implements the actual interaction/features with the cloud providers, and those licenses have not changed - it's just terraform core at this point.
So really you have until HashiCorp changes the interface between TF core and the providers, which would mean something like a Terraform v2.0.
This is an important point. The bulk of what matters isn't changing licenses. I wish this Terraform change hadn't happened, but in the scope of everything I don't think it matters to the vast majority of people. Unless you're building your product on top of Terraform, it doesn't make a big difference.
I overstated the timeline. The providers have minimum required Terraform versions, but I just checked the latest version of the AWS provider and it requires at least Terraform 0.13, which is over two years old now.
So really you have until HashiCorp changes the interface between TF core and the providers, which would mean something like a Terraform v2.0.