With Julia 1.6 now the LTS version and 1.0 end of life, supporting older Julia versions seems unnecessary. Why then 1.4 and not go straight to 1.6? Because 1.4 is the version shipping with Ubuntu 20.04 LTS and hence might still be relevant for some people. (Although it is better to use the official Julia build instead of the distribution provided anyway, but it's not ACME's task to push that.) Also, going to 1.4 turns out to be sufficient to drop dependence on Compat.
With Julia 1.6 now the LTS version and 1.0 end of life, supporting older Julia versions seems unnecessary. Why then 1.4 and not go straight to 1.6? Because 1.4 is the version shipping with Ubuntu 20.04 LTS and hence might still be relevant for some people. (Although it is better to use the official Julia build instead of the distribution provided anyway, but it's not ACME's task to push that.) Also, going to 1.4 turns out to be sufficient to drop dependence on Compat.