Plum trees can get various types of leaf spots. These, where the spot first browns and then drops out is common, but so are others.
Share a photo of the tree overall and estimate what percentage of the leaves are severely damaged. Did it look like this last year too? How is it doing in other respects? Heavy crop? Did it bloom well or have brown or early blossom drop?
How do you care for the tree? Has anything changed in the area, such as ditches dug nearby, or large tree removed changing the sun exposure.

The tree is wholly involved from the way it appears in these photos.
This Oregonian article features an Extension Service answer to someone with shot-hole. The links for more information in Heather’s answer still work, taking you to references for more details.
A photo with a few leaves, some top, some reverse, and your history of the tree fruiting and bloom may help us arrive at a more firm diagnosis.
If this is a high value tree in your landscape, consider hiring a certified arborist to make a site visit. If you will hire someone to apply sprays, contact them now to arrange it. Most fungal spray programs include applications before or as  flowers or leaves emerge, so the diagnosis needs to be made now when leaves are still healthy enough to see.