While explicit(false) does seem like a good way to indicate that, it seems like it's just a happy accident that it exists. The intended purpose of explicit(xyz) is that xyz is a compile-time constant that depends on the template parameters of the current (template) class. A literal true or false isn't needed by the language (just include explicit or don't), but the language would be more complex if they were disallowed so it's easier to allow them.

So in general, improvements to languages don't necessarily remove the need for comments over time.

(On the other hand, more radical improvements might remove needs for warning comments by removing the original problem. In this case, in a language where constructors were explicit by default and you had to opt in to implicit, you wouldn't need the warning to exist in the first place.)