This has always been the case with Ubuntu. Ubuntu only ever supported its main repository with security updates. Now they offer (paid) support for the universe repository in addition, which is a bonus for Ubuntu users, as they now have a greater selection of packages with security updates.

If you don’t opt-in to use Ubuntu Pro, nothing changes and Ubuntu will be as secure (or insecure) as it has always been. If you disable universe and multiverse you have a Ubuntu system where all packages receive guaranteed security updates for free.

Please note: I still don’t recommend Ubuntu due to snapd not supporting third-party repositories, but that’s no reason not to get the facts right.

Debian has always been the better choice if you required security updates for the complete package repository.

Personally I have my doubts if Debian actually manages to reliably backport security updates for all its packages. Afterall Eclipse was stuck on version 3.8 for multiple Debian releases due to lack of a maintainer …