I don't understand how anyone would trust a company whose main selling point is privacy when the same company is owned by a company that live off the opposite. Even if they could prove without a doubt to me that they can be trusted, that trust means nothing if you can't also believe that they are still trustworthy in the future. I personally don't trust either and haven't used them since they got bought up. Do you trust them?
I am left to wonder then, do you say the same of Brave? They are owned by an advertising company whose website once read "we know where your customers visit."? Or that Brave is funded by a Facebook board member? Or of DuckDuckGo which sends browser, country, and partial IP addresses to it's closest partner Microsoft?
The thing is, many privacy companies are owned or funded by those who do or have made money in surveillance capitalism, but you have to look at the companies history and it's actions.
Saying "how can you trust them because..." without taking a few minutes to do some basic research is really lazy and faux intellectualism.
When I was part of the PrivacyTools.io team, we heavily researched this and interviewed people involved and came to the conclusion Startpage hasn't changed their mission at all and in fact has been incredibly transparent about this partnership.
I would also recommend reading this interview with the CEO of System1 to better understand their decision to invest in Startpage and their views on privacy.