LetsEncrypt has been checking for DNSSEC since they launched 10+ years ago.

       The ACME standard recommends ACME-based CAs use DNSSEC for validation, section 11.2 [1]:
       An ACME-based CA will often need to make DNS queries, e.g., to
       validate control of DNS names.  Because the security of such
       validations ultimately depends on the authenticity of DNS data, every
       possible precaution should be taken to secure DNS queries done by the
       CA.  Therefore, it is RECOMMENDED that ACME-based CAs make all DNS
       queries via DNSSEC-validating stub or recursive resolvers.  This
       provides additional protection to domains that choose to make use of
       DNSSEC.

       An ACME-based CA must only use a resolver if it trusts the resolver
       and every component of the network route by which it is accessed.
       Therefore, it is RECOMMENDED that ACME-based CAs operate their own
       DNSSEC-validating resolvers within their trusted network and use
       these resolvers both for CAA record lookups and all record lookups in
       furtherance of a challenge scheme (A, AAAA, TXT, etc.).

Yes, that's my understanding as well. You'll notice my top-level comment from a few hours ago says that as well.

(You edited your comment to include more detail about when LE started validating DNSSEC; all I know is that it's been many years that they've been doing it.)