According to the BBC article:
Another traveller on social media said that staff at the airport check-in desk told them the new policy was only for the last 20 passengers checking in for a flight, and the charge had been introduced in the past few days.
A useful trick (as a pax) in the past was to check in shortly before the check in deadline, because by that time, only the best (most expensive) seats would remain available and you would often get an exit row for free. I guess Ryanair tries to make people check in early by this change.