On September 30, 2021, the older root certificate of Let’s Encrypt will expire (the
DST Root CA X3).
Here’s what happens after that date:
- Older devices that do not receive software updates will not trust the certificates from Let’s Encrypt anymore (for example, iPhone 4). This is because such devices will not have the new root certificate installed with software updates (the
ISRG Root X1)
- Modern devices already contain the new certificate,
ISRG Root X1, in their list of root certificates. No further action is required.
For more information on this topic, see the original update from Let’s Encrypt here: https://letsencrypt.org/docs/dst-root-ca-x3-expiration-september-2021/