> Are you sure that all the versions of Emacs that connect to
> elpa.gnu.org work correctly in the face of such a redirect? What about
> versions that donʼt support https?

I chose 26.1 as possible cut-off point as it defaults to building with
gnutls. Analysis of the GNU ELPA logs should show the percentages of
each Emacs version used. Once the percentage of requests from Emacs
versions below 26.1 drops to 1% or so, measures to stop supporting them
could be considered, such as forcing HTTPS usage.

Vasilij