$5 says the problem is the ad revenue they were living on has dried up and/or dramatically shrunk.
This has happened to a LOT of sites, and a more niche site like newgrounds probably doesn't command high rates, so they have to figure out how to fund themselves now.
Edit: Also, I don't think sites asking you to donate/pay for features is "enshittification". If anything, getting revenue from you, the user, is a hard counter to enshittification.
Enshittification is exploiting you for the benefit of capital: you become the product, and are pimped out to anyone who wants to pay.
If you're the one paying for the service, you're maintaining the relationship as a customer, and you keep the power in that dynamic, which is a power 'free users' on enshittified platforms don't have: you can take your money and leave, and that will actually hurt them.
This isn't globally true, of course - what is? - but telling you they need money to keep the site up and offer you a method to pay and provide something for your money is very very honest compared to what most of big tech pulls.