Sony has finally given us a definitive answer. It seems after all that panic-buying and speculation, their plans to 'close the PS Store on PSP' concerns in-game purchases or micro-transactions. You can still buy PSP content via PS3 and Vita. They've also extended the closing date to July 6th, instead of July 2nd. They published the following announcement on their site.

"What does this mean for you?
When the PlayStation®Store for PlayStation®Portable (PSP) was previously closed in 2016, you were still able to perform searches and make in-game purchases. Starting 6th July, 2021, you'll no longer be able to perform searches or make in-game purchases.

What about PSP content that you already own?
You'll still be able to download your previously purchased PSP content. You can download your previously purchased PSP content onto your PSP by accessing the Download List on the device.

What about PSP content that is available for purchase on the PS3 and PS Vita stores?
You'll still be able to purchase and play PSP content that is available on the PS3 and PS Vita stores. However, you'll no longer be able to make purchases via the in-game store for PSP content."