Do not restrict followers-based alias to members of non public channels.
Otherwise you may have issues defining a public mailing list with a
subscription step. People still wouldn't be able to post on a public
channel using a restricted alias.
Now that channels can follow documents, restricted aliases are a bit too
restricted. We consider that followers-based aliases accept emails from
the direct followers (message_partner_ids) or member of non public
channels (message_channel_ids, filetered public field). This way we avoid
having public channels added by mistake, leading to information leak.
Thibault Delavallée