What do I do with these counter blocks?
I know what they do.

(counter::control)
increment counter::control
clear counter::control

But what is it used for?

I tried to find a mention of counter blocks in the wiki, and was unable to do so. Is this an older feature that was removed?

GlueGourmand wrote:

I tried to find a mention of counter blocks in the wiki, and was unable to do so. Is this an older feature that was removed?

It is not removed. You can still find it if you search for it.