They used to never be visible, then became visible even if you didn't sign up for the Tech Preview. They probably just did a system clean up.
If you look at the note for the 2021.2 Tech Preview it says:
"As part of this effort, we will be resetting the existing Technical Preview permissions. This means that all access to older technical preview forum channels will be removed. That information will not be lost entirely. When you sign up for this new 2021.2 Technical Preview, you will once again have access to the prior forum channels as well."
They used to never be visible, then became visible even if you didn't sign up for the Tech Preview. They probably just did a system clean up.
If you look at the note for the 2021.2 Tech Preview it says:
"As part of this effort, we will be resetting the existing Technical Preview permissions. This means that all access to older technical preview forum channels will be removed. That information will not be lost entirely. When you sign up for this new 2021.2 Technical Preview, you will once again have access to the prior forum channels as well."
