Since every element of a website should be in a constant state of testing, establishing a new benchmark and then testing again (based upon whatever criteria/metrics are important for that website) therefore, I can’t imagine why A/B testing isn’t avaialable everywhere.
Of course, I could turn a webpage into a landing page test something for some period of time, pick a winner and revert the page back into a webpage but that adds extra steps to something that should be standard practice.
Always be testing.