Q.why are my 10-year old oleander plants dying?
I have ignored my oleander bushes for over 10 years, watering them very little and they still grew to over 10 feet. Now my entire hedge seems to be dying, starting in the middle for no apparent reason. What kind of pests do oleanders get and can I save any of them or should I take them out and plant new ones?

I believe your Oleander may have suffered leaf scorch.
I have listed a few links for you and how to care and prune your shrubs.
Use caution as all parts of the Oleander plants are poisonous.
https://www.gardeningknowhow.com/ornamental/shrubs/oleander/oleander-growing-tips.htm
https://www.gardeningknowhow.com/ornamental/shrubs/oleander/pruning-oleander-shrubs.htm
http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7480.html