Please read the following:
- Certificate Compatibility - Let's Encrypt (official compatibility of roots)
- Shortening the Let's Encrypt Chain of Trust (current and final timeline of legacy android support going away)
- Production Chain Changes (advice and troubleshooting for legacy support)
- Extending Android Device Compatibility for Let's Encrypt Certificates - Let's Encrypt (outdated by the above link, but provides the explanation better)
TLDR; older Android can be supported now using the "Long Chain", but you will not be able to use this chain after 2024-09-30 and must find an alternative solution before 2024-06-06.