... just increased. If I understand the recent summit agreement correctly, countries which leave the EU now lose certain rights of veto they previously had over transitional arrangements relating to their secession.

The previous treaties did not mention secession, and to date only Greenland, a dependent territory of the Kingdom of Denmark, has left the EU. Accordingly, normal voting rules applied when a country left. Now the bargaining position of a departing country has been weakened, increasing the cost of exit from the Union.

There is an oft-repeated fallacy, which I'd brand a lie if I weren't convinced that everyone who uttered it was ignorant, that it was impossible to leave the EU since the treaties didn't provide for it. It's frighteningly naïve to assume that the text of treaties determines what is and is not possible in practice, e.g., the invasion of Belgium in 1914 ... At any rate, the absence of explicit mention of secession didn't stop Greenland, and was often used, possibly unwittingly, as an argument in favour of providing for such explicit mention as part of a measure increasing the difficulty of its exercise.