TLDR: Ubuntu Pro offers additional security patches to packages found in the universe repo. Universe is community maintained so Ubuntu is essentially stepping in to provide critical CVE patches to some popular software in this repo that the community has not addressed.
I suppose it depends on how you look at it but I don't really see this as withholding patches. Software in this repo would otherwise be missing these patches and it's a ton of work for Ubuntu to provide these patches themselves.
Now is they move glibc to universe and tell me to subscribe to get updates I'll feel differently.
Debian includes ffmpeg, for example, in the main stable repo. Given Debian's reputation, I would think they are including these security patches in a timely manner, though I'm not entirely sure how to compare specific patches to verify this.
Of course, everything changes when you are selling support contracts. Canonical and Red Hat are the big two for enterprise because they provide support.
When I was last running Ubuntu on desktop, I signed up for an account and enabled these extra security updates. Yeah, it's "free", but it requires jumping through hoops. Requiring an account to get patches is the kind of user-hostile design pattern I expect from Apple or Google, but not in the desktop Linux world.
Nobody else has this hybrid model. RHEL is a paid distro in general. Most others are just free entirely. They all patch CVEs when they can. Ubuntu doesn’t write all of their patches or anything.
Debian's contrib repo, which is the equivalent of Ubuntu's universe repo, doesn't get security updates from the Debian security team, as it's not considered an official part of Debian. Package maintianers have to provide security updates. https://www.debian.org/security/faq#contrib
The difference is that Ubuntu provide paid support for contrib packages, including patches. Debian doesn't have any official paid support options.
No, they aren't. You can switch to their Universe patches anytime, at your own risk. If you want Canonical to mitigate that risk for you, you pay. Simple, really.
There are plenty of reasons to get rid of Ubuntu, but this isn't one of them.
Before Ubuntu Pro, packages in universe (and multiverse) were not receiving (security) updates at all, unless someone from the community stepped up and maintained the package. Now Canonical provides security updates for universe, for the first time since Ubuntu has been introduced, via Ubuntu Pro, which is free for up to five personal devices and paid for all other use cases.
Debian is actually not that different (anymore). If you read the release notes of Debian 12, you'll notice that quite a few package groups are excluded from guaranteed security updates, just like packages in universe are in Ubuntu. Unlike Ubuntu, Debian doesn't split its package repository by security support though.