google search – Run beta versions of sites SEO safely

We have and want to test a new site template on

What is the correct way to run a beta test without affecting SEO of the main

In the topic SEO safe way to run beta of new site on seperate domain I found 2 recommendations:

  1. Putting a robots.txt with noindex to our
  2. Canonicalizing to – Why? In any case we already have the noindex flag…

Does it still actual in 2020? And do we need to register in Google Webmasters Tools?