The ornamental plum is not a long lived tree in general. Most ornamental fruit trees start having issues around 20 years old or less.
We notice some spots on the trunk/branches and this looks like lichen and not a cause for decline. Here is more about it. https://extension.umd.edu/hgic/topics/lichens
We could not see around the base of the tree. The branch dieback may be due to root issues, planting too deeply, poor drainage, mechanical damage to trunk, possible borers, etc. Here is our publication on these types of problems
https://extension.umd.edu/sites/extension.umd.edu/files/_docs/programs/hgic/HGIC_Pubs/TreesandShrubs... and ornamental fruit trees https://extension.umd.edu/sites/extension.umd.edu/files/_docs/programs/hgic/HGIC_Pubs/TreesandShrubs...
All you can do is prune dead wood, water during dry periods, and make sure mulch is no thicker than several inches and away from the trunk.
The tree will continue to limp along and you will most likely have to consider removal in the future. Here is our publicaiton on When to Decide to Remove a Tree.