Old Android device can not connect to Web site or Mail service secured with LetsEncrypt
In case you get any complaints from web site owners with LE certs next year that some Android users can't access their sites:
The workaround is to install and use the FireFox browser, which maintains its own root cert store vs. what the Phone's OS provides.
This should affect only customers with web sites or webmail, hosted on Plesk servers and secured via LE certificates.
The other email services and Plesk Panel access are ssecured with HIS own certificates and should not be affected.
This is not an HIS.com or LetsEncrypt (LE) issue:
Old Android devices that haven't had OS updates since 2016 (and there may be a lot of those out there) won't be able to access sites with Let's Encrypt certificates in 2021 because the cross-signing agreement LE had with IdenTrust will expire and LE will use its own root certs from that point on.