The article described Thiel attempting to persuade people to leave the pledge, but not that he has actually succeeded.

This seems like the original article that is being quoted from. The quoted comments seem pretty bleak:

Thiel said he’s nudged a few to erase their signatures. “I’ve strongly discouraged people from signing it, and then I have gently encouraged them to unsign it,” Thiel said. Notably, in transcripts and audio lectures given by Thiel to Reuters last year, he recalled calling on the world’s richest man and soon-to-be first ever minted trillionaire Elon Musk to retract his pledge, warning the Tesla founder his wealth would go to “left-wing nonprofits that will be chosen by Bill Gates.”

Thiel said he’s had conversations with some signatories who have expressed uncertainty about their original decisions to commit. “Most of the ones I’ve talked to have at least expressed regret about signing it,” he said.

This is an individual who goes around calling Greta Thunberg the antichrist. I'm not extremely impressed by Thunberg, but I know someone with an ulterior motive who's playing on people's fears when I see one. Thiel's words should not be taken at face-value. He's not rejecting the effectiveness of giving, he's rejecting the very concept of good will.

My takeaways from reading the NYT article:

1. Social expectations have (at least in the U.S., which the piece mostly focuses on; it might be a bit different in Europe and other parts of the world) somewhat shifted away from supporting ultra-high-net-worth individuals from doing a lot of philanthropy and doing it publicly. This is sad. We should think if we could somehow help change that. People giving a lot of money to effective causes is something that definitely deserves to be praised and celebrated.
2. There is a lack of accountability for the pledgers. My guess is that Giving Pledge could benefit from scaling up their team significantly to contact and engage donors more regularly. On the other hand, I think society should hold the pledges more to account, and call out people who do not start donating.

Another, less plausible explanation for the slow-down in Giving Pledge sign-ups might be the growth of Founders Pledge in recent years. Some people who would have signed the Giving Pledge in the past might now sign the Founders Pledge instead.^[1] Anecdotal evidence from my own country supports this hypothesis: no person has signed Giving, but four billionaires / UHNWIs have signed Founders.

1. ^{^}

   Giving and Founders are not perfect substitutes; Founders also targets early-stage founders who are not yet (ultra) wealthy.)