The free Wildcard SSL certificate will be issued starting from the ACMEv2 environment. Wildcard certificates are more expensive ones than the regular single domain SSL certificates. The number of sites enforcing HTTPS increased at an impressive rate. Lets encrypt had a very good progress as it breaks the technical and cost barriers, We’re excited about the prospect of a 100% HTTPS Web and were working hard to get there the statement reads. To get the free wildcard certificate the domain validation needs to be completed using the DNS-01 validation method, this means you should modify the TXT records to prove the ownership.”]
Source: https://gbhackers.com/free-wildcard-ssl-certificate-lets-encrypt/