Acceptance is broader than people fear. Confirmed flights out of the country are the gold standard, but most countries and airlines also accept international train, bus, or ferry tickets that cross a border, since the requirement is proof you will leave, not proof you will fly. A booked but refundable flight counts, because it is a genuine reservation; you can cancel it after arrival, and many flexible travellers do exactly that.
What gets scrutinised: unpaid holds that were never ticketed, since check-in systems can see whether a booking is live, and third-party onward-ticket rental services, which airlines increasingly recognise. Anything of that kind carries risk at check-in.
A few countries want the onward travel within your permitted stay, with dates that make sense against your visa, so ensure the onward date falls inside your allowed days. Keep the proof printed as well as on your phone, and for overland exits, carry a confirmation with the border crossing clearly identifiable on it.