Almost 3 years have been passed since SSL was installed on miniBB primary domain, and since then, there were no issues for it (and also its issuer, Sectigo Ltd. — no remarks at all), but... this year, I've decided to try out Let's Encrypt.

The most impressive thing Let's Encrypt achieved in 2020, was the Android device compatibility extending. This made the Certificate-compatibility list much wider.

Actually, nowadays it covers most, if not all of the browsers and devices/OS miniBB could support, too. Even on iPad 3 (iOS 9.3.6) it still could run, if the Certificate is trusted (and then remembered) manually.

Who knows if much older devices should be even supported? Let's try and see — and Let's Encrypt.

R3 is not included nor listed in iOS 9 Trust Store, that's why ISRG is not sure of it in their official list. But it is surely amongst the "Always Ask" certificates as you mention, that ones "untrusted but not blocked". When one of these certificates is used, you'll be prompted to choose whether or not to trust it. That's the cite from Apple official.

Sad to say, Apple just releases and earns, but is abnormally weak in support their own inventions and you will never ever see them as lets encrypt supporters.