Charging more for transactions (in either direction) has nothing to do with "payments are costing more to process" lie.
This has everything to do with hook-and-charge business approach relying on majority of customers swallowing the arbitrary higher costs because it's more hassles to switch provider, especially for non-technical business owners.
There's intense competition for credit card customers that's driving transaction rates up in the US. Higher-end cards (Visa Infinite, Mastercard World Elite, Amex Platinum, etc.) offer a plethora of perks that are largely paid for by higher interchange rates on those cards.
My impression (ex Stripe with no inside knowledge) is that Visa has been raising rates. Stripe are doing their best to avoid passing it on. This is one place where it surely became egregious not to.
I’ve never seen a company as averse to squeezing users as I had when I was there. The finance guy I worked with produced analysis for the team showing ways to reduce pricing more often than ways to increase it.
Your skepticism is warranted against most businesses. From my experience as a PM at Stripe, it is unwarranted in this case.
Disclosure: ex Stripe, own stock. No inside knowledge.
According to the rest of the discussion on this page, Visa and other payment networks refund the interchange fee to payment processors (including Stripe) on refunded transactions.
There is no evidence that Stripe is any more charitable to their users than other payment processors. Stripe offers a commodity service, and increased the cost of using their service. Perhaps Stripe is "doing their best", but their competitors (e.g. Square and Amazon Pay) are doing even better by refunding the processing fee for refunded transactions.
So far said that the refund fees on the brand side specifically increased or came into existence. I don't know if they did or not. I imagine though that transaction fees rising (which people do mention in the discussion) has an effect here. If you were able to make a given margin before while still allowing refund fees to be returned. Now transaction fees increase. As a PSP you can either choose to increase your processing fees or find other ways to make even. If you can achieve that by not returning fees on refunds (and make a bit more money along the way) that makes sense to me.
My main point is that rising fees in general can be a reason here even if the card brands didn't specifically add a new cost to refunds per se.