The Brits that I know in various European countries have been dealt with quite civilly, as opposed to some of the people that I know that had moved to the UK where they find themselves suddenly without rights after being solid contributors to that society, sometimes for decades.
What makes you write this particular sentence?
This is a stretch by any measure. I'm an estern european in UK. Once I applied for ILR(Indefinite leave to remain) I can go and come with 0 problems. Not sure what "without rights" you are talking about but brexit doesn't bother me in any way.
We have plenty at work that arrived in uk around 2-3 years ago and they continue their lives without problems, go home for holidays and so on. Some work for us as self-empleyer/contractors. Not even those have any issues.
In theory, a Pole who applied for and got "settled status" in the UK can move around to work just as they could before Brexit. I have no idea how well applying for this works though.