The 63 character limit is in the DNS spec, not invented by Let's Encrypt. See https://stackoverflow.com/questions/32290167/what-is-the-maximum-length-of-a-dns-

The 63 character limit is in the DNS spec, not invented by Let's Encrypt. See https://stackoverflow.com/questions/32290167/what-is-the-maximum-length-of-a-dns-name https://stackoverflow.com/questions/32290167/what-is-the-max... for one explanation.