"l" not working to enter filter expression on mitmproxy 3?

I’m trying to use filter expressions on mitmproxy 3 and I’m not sure how to enter them. I’ve read that first I should press “l” and then I can enter the expression. But nothing happens when I press “l”

You are probably looking for f to filter the view. Docs aren’t updated for 3.0 yet, but you can view the current keybindings in the help (press ?).