Looks like they are giving 1 year before actually introducing that change. It's either the quote or the article wrong saying users will have only 4 weeks remaining.

Yeah, the article is wrong, you have 1 year to associate new custom domain addresses before the option to add is removed.

I wonder for how many it's more like 1 year to migrate to some other service.

They are not removing the functionality for people who already have domains added

on this note, what is the best tool I can use to start building my own subscribers database and sending email campaings? I've heard many opinions suggesting mailerlite, any thoughts?