When upgrading from CA Easytrieve release 11.0 or 11.5 to release 11.6, is it a requirement to recompile all of the
CA Easytrieve programs?

Release:
Component: EZTPLS

Recompiling every program is not necessary as long as maintenance is applied and some testing is completed and is successful. The same compiler is used between all release 11.x releases.

Here is the link in DocOps: 
https://docops.ca.com/ca-easytrieve/11-6/en/release-information/new-features-and-enhancements