I cannot find any hard evidence of this claim -- I don't have reason to believe you're making it up, but I also would expect this change to be more widely announced. The best I can find is some discussion by Let's Encrypt staff that the roots want to stop issuing clientAuth-enabled leaf certificates eventually. However, there haven't been any hard timelines established because (at least) some mail servers in particular are using domain-validated public certificates for opportunistic mTLS.

I've scoured the CA/Browser Forum BRs and ballots, Chrome Root Store policies, and CCADB policies, and can't find mention of this coming restriction.