I found a solution. This response summarizes the advice I got:
After choosing Interface > Default (not Classic) style, the “New” button at the very top of the screen still produces a new post in the block editor. That’s the button I’ve always used. I hardly even noticed the “Add Post” button. But with Default, that button does give me a choice of Block or Classic editor, and those two do deliver the different types of new post that one would expect. Apparently the Default choice is also responsible for restoring Classic Editor as an option when mousing over the title of an existing post in the list of posts. Not sure I understand the wording – Default vs. Classic – but it works.
