In September 2021 Chrome will begin displaying full-page warnings when visiting websites that don't support HTTPS. Mozilla has similar plans for Firefox.
Revue subdomains still don't support HTTPS. The only workaround is to transfer all of a domain's DNS to Cloudflare, which may be impractical.
Revue has been meaning to fix the issue for over a year but the clock is ticking. If you're concerned the lack of HTTPS support for your domain may harm your newsletter, be sure to get in touch with Revue and let them know.