Page.title(as_url=True) currently does not encode the slash (/) character, because that is a default exception in the underlying quote_from_bytes.
Unencoded slashes that are part of page title (e.g. "A/B_testing") are not a problem in Wikipedia article URLs, but it breaks the REST API that makes extensive use of the slash to separate its arguments. Accessing https://en.wikipedia.org/api/rest_v1/page/html/A/B_testing results in an error, while https://en.wikipedia.org/api/rest_v1/page/html/A%2FB_testing works as expected.
On the other hand, I have not identified any case in which escaping a slash in a page title embeded in a URL would break existing functionality.
Therefore I suggest removing the slash character from the exception list, basically by adding safe='' as the second argument of quote_from_bytes.