The millennium bug turned out to be largely a non-event because of a great deal of work by programmers to check software and rewrite it where necessary.

The millennium bug -was it a myth?

I know people in IT who confirm this from their own experience.

The Brexit equivalent would be to leave with a deal. I've no idea whether or not that will happen, but if we don't then we can expect all the issues raised in this thread to take effect. I certainly hope, and indeed expect, that in time the UK will be able to agree new arrangements with the EU and that these will include arrangements which will allow musicians to tour with the minimum of additional bureaucracy and cost. However no one can say how long this might take, and I also expect that the needs of musicians and other performers will be a low priority compared with maintaining supplies of food and essential medicines.