So now I wonder why Angular team decided to use it? And what does it
mean?
Yes, it marks methods as internal/private. Here is the quote from the article that was written by Uri Shaked who got this information from the Angular core team members:
The first thing you many notice is a lot of obscure method names,
starting with the letter ɵ (Greek Theta) followed by 3 other English
letters (e.g. ɵvid). The letter ɵ is used by the Angular team to
indicate that some method is private to the framework and must not be
called directly by the user, as the API for these method is not
guaranteed to stay stable between Angular versions (in fact, I would
say it’s almost guaranteed to break).