Is it right to assume that around 80% of the questions in the 1Z0-819 exam will be from relatively new topics (introduced from Java 8 onwards) like lambda, try-with-resources, var, modules, security and so on?
No. There's a lot on existing topics too. Also, remember that you need to know multiple topics to get a question right. So if you know streams well and not exceptions, you can still get the "streams" question wrong.