According to the root certificates page (Chains of Trust - Let's Encrypt) , "Let's Encrypt R3" is listed under the section entitled "Retired: These intermediate CAs are no longer being used to issue Subscriber certificates."
However my Let's Encrypt client (LEGO) appears to be happily issuing new certs signed by R3 ?
I mean, I think they could have phrased things a little clearer, having all the intermediates together rather than one section up top only talking about the after-June-6 configuration, and then basically another whole page just sitting below it for the existing chains. But they wanted to just be able to delete the bottom part once they deploy the change, which I can understand.