The old “Indefinite” award wasn’t actually indefinite, it was an award without a fixed end date and could be reviewed at at any time the DWP chose to.
With PIP the nearest thing currently is an award for “An ongoing period”, this is meant to be 10yrs with a “Light touch” review at the end of that period. As this was only recently brought in nobody with the award has been on it long enough to say how it’s working in practice.
Please excuse spelling/typos. Apart from being a clot it turns out I had one on my cerebellum that's now causing various problems!