That's a side project, and doesn't explain all the services that use it without images.
Regardless, you always have to connect to 3 different domains - typically the website itself, google.com and gstatic.com. These 3 domains allow for very accurate triangulation across the internet, and each one will fingerprint your browser. They might not know your bank account number or social media account name, but they know that someone using your browser banks with this bank and has an account on that social media, along with thousands of other data points.
It’s so obvious it’s a scam too because of how vague some of the “challenges” are. “Click all the squares that contain a bike” and then you do it in every conceivable way AND IT STILL WONT ACCEPT YOUR SUBMISSION. Stupid
I have a suspicion that it might depend on a few factors that might be related to Google’s ability to track, such as the browsers you use and whether you’re on VPN. I saw a post on Lemmy the other day (can’t remember where) that they suspect captcha is more annoying on Firefox, whereas fine with Chrominum-based browsers, esp Google Chrome. I think I sometimes experienced this as well, but I didn’t keep track.
I had one today where it was click on all the squares that contain a bus. One square very clearly contained a truck, not a bus. When I didn't select that square, it told me I was wrong.
I've always thought that CAPTCHA also checksfor time, e.g. if the correct solution is entered too fast, it will determine that you are a bot. This would mean that a fast bot would not really "win". Not sure if this is the case or why I've thought it is, but it came to my mind from this 😅
The captcha with the checkbox tracks a lots of items like mouse movement, browsing history etc. Full list is not public but they use all their spying tools on that one
When I added the contact form on my website, I wanted to add reCaptcha so it would protect me from spam emails and make it easier for people by not having to click on pictures of fire hydrants. Reading a little about the privacy concerns about reCaptcha, I dropped it all together.
I am in the EU, have a small numbers of visitors, it'a B2B thing so it was not worth it.
I've seen those puzzle piece captchas (common on crypto exchanges) that always say I completed the puzzle faster than 99% of users (even if I didn't do the puzzle particularly fast). I presume those 99% are all bots attempting to hack into crypto wallets.