I am a little suprised to find google captchas on this site. Especially as it is a site about FOSS. It is just another of their tracking mechanisms in a somewhat useful form. And it is not practical, either: maybe because I am using a VPN I have to enter a captcha on every message.
As a question:
Is/has spam (been) such a big problem on this site, that is necessary to give personal information about your visitors to Google?
Ok - you find that strange (and I agree to some extend, why every site requires/enforces links to other sites), but what is the question (and by signing up you agreed to https://www.libreoffice.org/about-us/privacy/privacy-policy-en/ - 1.2.Use of reCAPTCHA)? (Personally, I even can’t remember, when I had to enter a captcha for the last time).
As with any captcha, the goal is to fight spam. As with any captcha, it is inconvenient. I believe that it stops showing after several posted questions / answers, or maybe with some karma level, and is not related to vpn.
Is/has spam (been) such a big problem on this site, that is necessary to give personal information about your visitors to Google?
Yes. This site is driven by community. Me personally, along with other users, in our spare time, spend time reading the spam questions, ant tell them from valid questions like yours, and manually remove them. And spam is still considerable problem here, even with captcha. You are not “my” visitor (in that sense it’s power users of this site who the captcha helps), so it’s not like “we are giving something about our visitors”. Either there is a machinery helping us, or there won’t be any site at all.
to give personal information …
I’m not a fried of that at all, but this statement lacks also some reasoning if taking the definition of personal information into account. You visit this site via VPN (as per your own statement), you have a nickname here, no email address is visible to any member and your profile is almost empty - so what personal information does this site have about you?