Hello,

I think a mature tree such as this is worth trying to save, especially when you think about the years it will take to replace such a tree, or the amount of money it will cost to remove.  One way you could go is to consult an arborist to come to your site and take a look at what is going on.  We always recommend consulting an ISA-certified arborist as they go through more rigorous training.  One in your area can be found on the ISA website:  Find an Arborist (treesaregood.org) A good arborist will tell you whether a tree is worth the investment of pruning before they do the work.  

I would also recommend removing the sod around the trunk of the tree and locating the root flare of the tree.  This is where the trunk widens right above the root system.  I am wondering if soil was added over the root system when the retaining wall was put in.  It seems the tree is buried too deeply which can cause this kind of cracking and branch die-off.  Once this root flare is located, a few inches of mulch can be put around the tree, only a few inches and not touching the trunk.  This helps keep the roots cool and moist while allowing for air exchange. Make sure the soil is graded so the tree does not sit in water when it rains.  Arborists can use an air spade to remove excess soil from root systems if this job seems daunting.  The other thing I would recommend is watching how much rain we are getting.  If the current drought pattern continues you should provide a few inches of water for the tree every 3-4 weeks. 

Here is an article about trees being planted too deeply, and another about how to water mature trees.

https://www.canr.msu.edu/news/planting_a_tree_successfully_requires_the_correct_planting_depth

https://extension.umn.edu/planting-and-growing-guides/watering-established-trees-and-shrubs#how-much-water-to-use-1261761

I hope this answers your questions.  Thank you for contacting Ask Extension.

Ruth