In January 2018 The Netherlands introduced an airport transit visa requirement for Cuban citizens travelling to a destination outside Schengen via the N.L., leaving Paris as the only Schengen airport that Cubans can transit visa-free. - What is the most reliable source to check up-to-date requirements before booking a flight? - How much notice is given of such a change, and what happens if the traveller has already bought their flight ticket and there is insufficient time to apply for a transit visa before the departure date?
The most reliable source is going to be each particular country's customs and immigration site. It will be the most direct message from the source.
As for notice, most countries do not change such rules immediately and give a reasonable adjustment period, but in the case of significant amounts of abuse of their rules, it is possible the rules could change rather quickly. I expect you'd get some consideration if you tried to enter under such conditions, but there are no guarantees.