Is there any way to let unregister users create new posts or answer existing?
I want to have this kind of mode enabled for the first time. The reason is, often times people refuse to post anything when they see that they need to register
That's a risk I wouldn't take personally.
It is Ok, I am ready to take a risk at this moment. Once I have some initial content, I will disable this.
I am going to send a link to my forum only to specific people for now. So, I may even consider blocking it from search engines to avoid spam.
Is there any way to do it?
Bots don't use search engines, they follow links. Unless you have some sort of captcha to stop the bots, you will get lots of spam.
This is only a theoretical question. There is no functional plugin that would allow anonymous postings.
I have captcha and believe me they will find you! They don't have to follow anything or use search engines they scan the entire IP address range of the internet over and over and over again. They are the search engine for themselves and as soon as they see a form, bots go at it like hungry lions when meat is thrown in their cage. I get about 50 registrations a day by bots that get past my Captcha from Google. It's not hard to do this at all. Your forum would be so swamped with SPAM you would not be able to keep up with it. This is NOT a good idea at all.
Maybe, To get some initial content:
Search the net for "http basic auth" / "htpasswd"
All my test sites are behind such passwords which makes them inaccessible for everyone except those who have the password