There's a bit of confusion among the gurus in #AskUbuntu and #UbuntuForums about how the esm repos have changed with the replacement of Ubuntu Advantage by Ubuntu Pro.

As far as I can tell after decrypting the marketing-speak, it seems to look like this AskUbuntu answer: https://askubuntu.com/a/1453618/19626

However, correction or confirmation from somebody inside who actually knows the product would be welcome.