When trying to access a site on an older version of Android or Windows, a warning appears e.g. "Your connection is not private"
SNI (Server Name Indication) is a newer technique of serving SSL certificates that powers Heroku SSL. However this isn't supported on some older clients, as described here: https://devcenter.heroku.com/articles/ssl#minimum-supported-browser-versions
If you need to maintain support for older versions of Android and Windows then we recommend the legacy SSL Endpoint add-on https://devcenter.heroku.com/articles/ssl-endpoint