Nobody gets "put on" PIP. They will be invited to start the claim process via letter and you then get 4 weeks to start it off with a phone call. If you don't make the phone call then DLA stops 8 weeks later after 1 4 week warning/reminder. They're currently working through the aged 50+ indefinite awards although this has either slowed or even stopped in some areas thanks to backlogs accruing.
A number of people report first knowing they'd been invited when their DLA stopped i.e.they didn't get the letters so benefit stopped. So, it's as well to ensure your 4 weekly payments are still happening regularly as there's no backdating.