I mean, why wouldn’t you demand proof? Governments have a long and nasty history of conflating their political interests with security interests, just trusting their assertions seems extremely foolhardy.
More succinctly; “it would compromise means and methods” is a phenomenal way to cover up shaky or non-existent evidence.
Obviously banning an organization is much lower stakes than killing or jailing someone, and we should adjust accordingly. Still, if Israel (or any other country really) is going to run around and say that an organization or person is connected to terrorism, they should pony up enough evidence commensurate with the actions they propose so that we can discuss the matter. The pressure for any state to conflate political and security threats is high, and public scrutiny is part of what provides a counter balance.
This is particularly important if a country is demanding that the international community writ large take action against a given person or organization. Should not the citizens of the countries being asked to participate have the opportunity to see some evidence and discuss the matter?