I can't speak for the spec authors, but IMHO, tags should be deprecated and eventually removed when they are deemed to be useless, especially when their functions and/or semantics are covered by another tag, and especially when their use is harmful (or rather, more harmful than beneficial).

In my (very personal) opinion, an HTML tag or attribute, and more generally a feature of any design/development framework, should be considered possibly harmful if it:

- presents possible security problems; for examples, consider some of the points listed here: https://html5sec.org/

- promotes poor usability or accessibility; e.g. interactive tooltips with links or controls in them, for example, are quite difficult to make accessible, and I wouldn't want an HTML <tooltip> tag without a lot of discussion about accessibility

- promotes anti-patterns; e.g., at this point I think <marquee>-style scrolling informational text is an anti-pattern in a web context, since it can the text much harder to read, especially on small screens

Of course, none of these concerns should lead to immediate removal of a thing as soon as they're pointed out, but they should be discussed and considered. It's a cost-benefit analysis: what does this feature actually buy us that isn't easily achievable with other features, what problems is it causing and how severe are they, and are the benefits worth the problems?

As for <menu>, my guess, though I haven't been able to find the actual discussion, is that it was removed because its semantics are somewhat in conflict with <nav>, and probably its most common use was custom context (aka "right-click") menus, which bring a lot of accessibility problems with them. I don't know that I agree with the decision to remove it altogether, since I think its use to semantically identify and group web application controls is very valuable and not covered by any other tags (though I'd love to be corrected), but I do think that context menus, which to me seems like the most common use for the <menu> tag, are a very problematic design element. Again, it's a balance; is it worth the problems it causes? I guess the authors decided it wasn't.

(Just to reiterate, I don't know why <menu> was removed, I'm just guessing. If anyone can find any of the discussions about <menu> and the problems with it, I'd love to read more.)