Bottom line: Sony doesn't seem to be preparing to remove the ability to buy legacy digital content for PlayStation 3, PlayStation Vita, and PSP again (not yet anyway). It is, however, adding another hurdle to go through for those who are still buying things through those systems.
On October 27th, Sony will remove the ability to add funds to wallets on the PlayStation Store via a PS3 or Vita. Customers will no longer be able use payment methods like credit cards or PayPal directly on those systems.
Users will still be able to buy and download things on those stores like PS3 games, Vita games, PSP games, digital original PlayStation games, or digital PlayStation 2 games. They'll just have to add funds through either a web browser, the mobile PlayStation app, a PlayStation 4, a PlayStation 5, or gift cards, before buying items for older systems. This also applies to buying things in in-game stores. Vouchers for products and subscriptions will also still work like this.
In March, Sony announced it would completely shut down the digital PS3, Vita, and PSP stores due to the challenges of keeping those stores running on aging hardware. After a lot of criticism from users who still wanted to buy older games from those stores, Sony stepped back from closing the PS3 and Vita stores, but still closed the PSP store in July.
Customers can still buy PSP content through the PS3 and Vita stores, then download it on a PSP or transfer them to a PSP from a PS3. Late last year, Sony stopped selling PS3, Vita, and PSP games through the web browser and mobile stores. Sony hasn't said anything about how much longer it plans to keep selling those games through PS3 and Vita.