Nobody is setting up "privacy-friendly" frontends to track browsing data that they couldn't otherwise get without access to Google's/Twitter's/etc. logs? Because I think they are.

Nothing. An acquaintance of mine develops a third-party frontend explicitly marketed as a privacy-friendly alternative and actively looks at lots of user data (which includes the full name) without disclosing. I honestly believe that it's only done for improving the service (and it helps tremendously) but I can't get through with arguing that this should be transparent.

You could notice by closely reading the source code.

How could you ever prove that nobody is doing that? You can believe anything that way

One can't prove god doesn't exist either, but as someone who made some privacy-friendly front-ends, I tend to expect honest intentions. If you find one that suddenly asks for your login data or sets tracking cookie, sure, be wary, just as with any other site that asks for data they don't need (see: literally every cookie wall, because if they had good intentions, it would fall under one of the five other reasons to use personal data and they wouldn't need to fall back to asking for consent)

Don't use it. Stop shitting on everything you disagree on. Besides, privacy is not black and white. No one is implying such a ridiculous claim. Just because you grew up in a disgusting for-profit driven web, doesn't mean that everyone is trying to get you. Believe it or not, there are people who actually value privacy and actively voluntarily support decentralized and non-invasive parts of the web without hoping for any incentive. Besides, majority of private frontends are extremely fast and loads in an instant, which saves a lot of time

Yeah, the possibility of any of them being a honeypot I'd say is real.