There is a lot of mojibake on Mailman. You can see it on listinfo for several lists, like "WikiTe-L", "Wikipedia-pt-saude", "Wikipedia-AS", "Wikipedia-BN", etc. I noticed this because I am an administrator of the list "Wikimedia-no", and even the mails it sends out have mojibake in them.
The list I admin is very low-traffic, so the last requests to join (which generate emails to me to approve/reject) were in May 2019, and they were fine. Then on 28 August this year (and every day since), there has been mojibake in the emails I get from this list. In other words, the problem arose some time between May 2019 and August 2020 – I wish I could be more specific.