Countdown to the expiry of the R3 (cross-signed by IdenTrust) certificate

On Sep 29 19:21:40 2021 GMT an intermediate certificate by Let's Encrypt called "R3" will expire. There are two versions of R3, one signed by Let's Encrypt/ISRG and one signed by IdenTrust. While the Let's Encrypt version of R3 still has about 4 years left of lifetime, the IdenTrust version expires very soon. This is related to the expiry of DST Root CA X3 itself.
You should not use this intermediate certificate anymore.
R3 cross-sign has expired!