Just a heads up about Let’s Encrypt and old Android versions:
Some software that hasn’t been updated since 2016 (approximately when our root was accepted to many root programs) still doesn’t trust our root certificate, ISRG Root X1. Most notably, this includes versions of Android prior to 7.1.1. That means those older versions of Android will no longer trust certificates issued by Let’s Encrypt.
There will be site owners that receive complaints from users and we are empathetic to that being not ideal. We’re working hard to alert site owners so you can plan and prepare.
My old phone has become my new phone when I flashed it to LineageOS 17.1(Android 10) A new battery is on it’s way so the less-than-1-day operation time will be brought up to 3 days again.
(btw, I have a Xiaomi MI 5)