HEX_MIRROR is being deprecated because it was being used for both the Hex repository and build artifacts.

Going forward you should use HEX_MIRROR_URL or HEX_TRUSTED_MIRROR_URL for the Hex repository (repo.hex.pm). HEX_TRUSTED_MIRROR_URL should only be used if it’s a mirror you can trust with secrets such as authentication credentials and private packages, otherwise use HEX_MIRROR_URL. You can use these if you are on the latest version of Hex.

For build artifacts you should set HEX_BUILDS_URL, this variable is used by mix local.hex and mix local.rebar. This variable is being introduced in Elixir 1.15.0 so it’s not available yet. To avoid the error in the mean time you have to set HEX_MIRROR only when you call the mix local.* tasks. But since Elixir follows HTTP redirects you don’t have to make any changes, when repo.hex.pm changes to permanently redirect to builds.hex.pm Elixir will continue working.