It's pretty easy to find articles confirming the Musk/Signal thing https://duckduckgo.com/?t=ffab&q=musk+signal&iar=news&ia=news
Of course most of the rest is speculation, but... the article seems honest enough about it?
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IDK about each specific requirement (especially the "inactivity" one, but ... dude, just log in every 6 months), but I'd say a lot of "privacy" email providers should meet your needs.
Try looking into Proton mail (warning: you'll have to setup a local relay if you need IMAP/SMTP) or maybe search the web for something like "privacy email provider free" and look into the results.