Starting with Chrome 69, logging into a Google Site is tied to logging into Chrome.
If you are a chrome user, you have probably seen that since a week (more or less) your chrome automatically updated to a new version (as always, without your permission, for your safety…). You may have noticed also that you don’t need to log into your browser anymore since the browser automatically logs into your Chrome account whenever you sign in to any Google service.
This “feature” has not been received very well from the developer community, and there has been a ton of complaints and a lot of people moving away from Chrome to other browsers or recompiling some version of Chromium without the Google-services features.
This article explains very well the problems, but what is more interesting (from my point of view) is that less than a week later, Google officially responded to all the critics
with an article explaining the changes and also that it will be an option to disable this feature in the future version of Chrome.
I have never thought that Google would have responded to some developers complaining about it, I would have guessed they would ignore all the complains (like they already did many times) and move on with their own schedule, but I was surprised by their statement.