holoduke parent
Older Android 7 devices are not supported with letsencrypt. For us still 20% of our userbase. We went for a paid subscription with zerossl.
It's likely to get worse as CAs rotate roots more frequently. Cross-signing will work for a time (provided you correctly install) but at some point, older devices will drop out of support and that'll be it.
That's six years obsolete, without security patches. What sector has 20% of users with that hardware?