Changing the Terraform backend from the default "local" backend to a different one after doing your first terraform apply is:

Mandatory is incorrect; many valid workflows use the local backend throughout a project's lifetime, particularly for personal or single-user projects.

Terraform allows you to change the backend configuration after infrastructure has already been provisioned. Running terraform init after updating the backend block prompts Terraform to migrate the existing state to the new backend, making the switch optional and reversible rather than mandatory, impossible, or discouraged.

Impossible is incorrect; terraform init handles state migration when the backend block is changed, making the transition straightforward.

Discouraged is incorrect; migrating to a remote backend is actually encouraged as a best practice for collaboration, not something to avoid.