(new mw.Uri('https://en.wikipedia.org/wiki/Main_Page#a/b/c')).toString()
// "https://en.wikipedia.org/wiki/Main_Page#a%2Fb%2Fc"

The slash is a valid character in a fragment and there is no reason to encode it; a fragment with a raw slash and a fragment with an encoded slash are different for most purposes, such as navigation or programmatic equality checks, so e.g. location.href = uri; and location.href = new mw.Uri( uri ).toString(); will result in different behavior if uri has a fragment used for anchoring or routing.

The same problem exists with various other characters, but with the slash it is especially bad since it's used extensively for routing in MobileFrontend. So, any kind of URL manipulation, even if completely unrelated to fragments (such as adding or removing a query parameter) will break MobileFrontend routes.

See also T103379: Add 'url' module for native URL constructor (with conditionally loaded polyfill) which proposes a new class with spec compliant behavior.