From today forward, new Let's Encrypt certificates will be signed using their new R3 intermediate certificate instead of the X3 intermediate certificate. Most users won't notice it, the transition is really smooth. If you have 2 1 1 or 2 0 1 TLSA records set for one of your services with a X3 certificate, make sure to create extra TLSA records for not only the new R3 intermediate but also the other intermediate certs to prevent any downtime of your services: http://dnssec-stats.ant.isi.edu/~viktor/x3hosts.html See https://letsencrypt.org/2020/09/17/new-root-and-intermediates.html and https://community.letsencrypt.org/t/beginning-issuance-from-r3/139018/2 for more information.
How do I know if I have to do something or can just do nothing? I did read through all three links provided.
Do you have any TLSA (also known as DANE) records set for your services? If not, no action is needed.
