The ports that receive -stable updates in CVS. That usually means important security fixes, chosen at the discretion of both the port maintainers and developers building stable packages. The @OpenBSD_stable Twitter account may be helpful, ports tagged OPENBSD_6_5 are stable updates, and should receive updated packages.
Is this the same as what mtier provides ? Or are the packages different. I was trying to find chromium as that is about the only internet facing package I would use in a hypothetical desktop setup. I didn't see it on the link you posted.
Unrelated. I don't know what all mtier provided, but chromium port updates are not backported to -stable. A quick check indicates they did not provide this.
Also note that mtier announced they will no longer be providing -stable packages of their own.
If you want the latest chromium, you need to run -current snapshots. Packages are available. Chrome is enormous dwarfing the OpenBSD kernel and base system combined and is under constant development, it's a challenge to keep updated on -current, let alone attempting newer versions on -stable.
https://twitter.com/openbsd_stable
Also a full list on the mirrors.
https://cdn.openbsd.org/pub/OpenBSD/6.5/packages-stable/amd6...