Centos 8 Stream EOL

Centos 8 Stream is a separate thing, not really trying to be a direct equivalent of RHEL 8 (not in terms of the packaged software or timeline).
Centos 8 (non-"stream") was supposed to track RHEL 8 and presumably would have matched the RHEL 8 EOL date if the project hadn't been killed off.

From the Centos FAQ (emphasis added):

Q6: Will there be separate/parallel/simultaneous streams for 8, 9, 10, etc?

A: Each major release will have a branch, similar to how CentOS Linux is currently structured; however, CentOS Stream is designed to focus on RHEL development, so only the latest Stream will have the marketing focus of the CentOS Project.

Because RHEL development cycles overlap, there will be times when there are multiple code branches in development at the same time.This allows users time to plan migrations and development work without being surprised by sudden changes.

Specifically, since the RHEL release cadence is every 3 years, and the full support window is 5 years, this gives an overlap of approximately 2 years between one stream and the next.

Ie, the Centos 9 Stream should appear when RHEL 9 releases and coexist with Centos 8 Stream until RHEL 8 Full Support ends.
My understanding is that if you run Centos 8 Stream, you are expected to transition to Centos 9 Stream during that overlap (while RHEL 8 continues to be maintained in it's Maintenance Support phase for another 5 years after that time).

What will happen is RHEL 8 Full Support ends, which is the EOL for CentOS Stream 8.

CentOS Stream versions do not have a 10 year life, more like 5 years. As the testing repo for RHEL, Red Hat wants you to keep QAing features, and has no interest in providing 5 additional years of security updates for free.