Both will probably not work for my (figurative) grandmother, sending an email to the address I gave her to reach the Discourse category replacing a mailing list.
Such workarounds are out of my experience often rather difficult for users with limited IT literacy and then suddenly things get rather complicated (cf. Incoming emails trimmed despite trimming disabled - #9 by sjjh) and people quit using Discourse and fall back to Facebook Messanger or the like because its “so much easier”. Which is a shame IMHO.
And as I hope/believe that this is not the case, I’m also wondering what potential harm is feared to allow for this (admin) configuration to include “0”.