I just learned on Erlang OSS Discord that erlang.mk will no longer embed a package index (except for a few selected packages used for plugins and tests, as well as ranch and cowboy).
I guess the reason for this step is because such an index is hard to maintain and practically always out of date for the indexed packages.
What this means is that, when using the latest erlang.mk, you will have to add corresponding dep lines to the Makefile, for example:
DEPS = mysql
dep_mysql = git https://github.com/mysql-otp/mysql-otp.git 1.8.0