Others may wish to comment in more detail. But, I think you should review the below two topics to better understand the Let's Encrypt support for older Android and upcoming changes in that regard.
DST Root CA X3 Expiration (Sept 2021)
Pay special attention to this section and related links in that page
DST Root CA X3 will expire on September 30, 2021. That means those older devices that don’t trust ISRG Root X1 will start getting certificate warnings when visiting sites that use Let’s Encrypt certificates. There’s one important exception: older Android devices that don’t trust ISRG Root X1 will continue to work with Let’s Encrypt, thanks to a special cross-sign from DST Root CA X3 that extends past that root’s expiration. This exception only works for Android.
The DST cross-sign is expiring this year. The below topic explains this