Saying, ‘ah but did we know?’, is ridiculous. The proscribing was you being told. That’s what it’s for.
We as individuals don’t know all sorts of things, sometimes because it’s deliberately hidden by the group or the authorities.
So when a group is ‘proscribed as a terror organisation’, you should be pretty damn sure of your facts before you decide to support them anyway.
If You chose to support people who did this despite knowing they were proscribed, It’s on you.