We used to have a "plus" repository for CentOS 7 and 8, that contained packages from OS, but rebuilt differently. Most known examples in CentOS 8 plus repository are the kernel-plus and thunderbird (with openpgp support).

As CentOS 8 is going EOL end of this year, and that some us still need to rely on that specific thunderbird package, we agreed to just use the SIG process for the plus repository, instead of having such packages being built through the distro builders.

So if you're already on CentOS Stream 8 , you can just install (like for other SIGs) a specific package containing both the .repo file *and* needed gpg key used to sign the packages.

How to enable/use it ? :

sudo dnf install centos-release-plus -y

As stated in /etc/yum.repos.d/CentOS-Stream-Plus.repo, you can now enable it (remember that it can overwrite base packages, reason why we also ship it as disabled)

sudo dnf config-manager --enable centos-stream-plus

You can now install pkg from it, like thunderbird :

sudo dnf install thunderbird

Tip: if you want to just use some pkgs from one specific repo, you can also just use excludepkgs/includepkgs statements in your .repo configuration (see https://dnf.readthedocs.io/en/latest/conf_ref.html#options-for-both-main-and...)