I am including dividends.

I sometimes wonder if there are some long forgotten brokerage accounts just idly compounding.

There aren’t, actually, at least not in the US. When an account has no activity on it for long enough, it gets liquidated and transferred to the state until someone with a rightful ownership claim comes forward to claim it. This is called “escheatment.”

https://en.wikipedia.org/wiki/Escheat

Though dividends paid in cash don't really count as compounding no?

You can reinvest them it or he stock after you’ve paid income taxes on them.