Let's Encrypt had proposed changes starting on January 11th, 2021, that would no longer offer SSL support for Android hardware running releases prior to Android OS 7.1.1. However a recent update from Let's Encrypt outlines their plans to extend support for Android Device Compatibility for Let's Encrypt Certificates for a further 3 years.
This means that Heroku ACM will continue to work with older versions of Android for the next 3 years, and customers will no longer have to move to self managed certs in order to have compatibility with older versions of Android.
Full details from Let's Encrypt can be found here: https://letsencrypt.org/2020/12/21/extending-android-compatibility.html
In the case that there are any further changes to this situation, we will post updates update as they arise.