I've tried searching Trenitalia's website and can't find up-to-date info on whether the regional train tickets are still flexible to be used within two or three hours of printed departure time. Does anyone know whether this has changed (e.g., due to COVID)?
With the relevant exception of the Cinque Terre, Regional train tickets purchased on line can be used on the selected train and on a similar one departing within the following four hours.
It's explained here: https://www.trenitalia.com/en/purchase/regional_online_ticketandpasses.html
You can't use a Regionale ticket on a train with reserved seats, even if it makes the same route you paid for. There is no need to print-out and/or time-stamp Regionale tickets purchase on the official site: you can show the conductor your mobile phone.
OK, thanks! Looks like it's still going, then. Perfect for day trips on regionale routes.