I actually loathe those "approx whatever time to read" notices for about the same reasons the OP lists.

You or your model do not have and can not a clue how fast or slow I read, or, and that is the point, how much time I intend to spend on whatever is up.

The mirror is that you cannot know who my recipent is, or what I'm trying to communicate. It is equivalent in this sense.

You only [propose to] clutter the already overcluttered interface with crap, slop and shit. So bugger off pretty please. If you do not, there goes your product: outta my window.

I think it’s different when it’s shown to the author, not the reader. It’s basically a word count feature, which has been useful since forever. Except it doesn’t translate into a unit that really means much for this context.

Yes yes yes, it won’t be one size fits all and all those uninteresting “but what if…” points.

What we want is to cue the slop generator just what they’re producing for their coworker or whoever.

I hate getting huge pages of careless slop that the unthinking author probably imagined would look impressive.

Maybe only show it as a result of the user pressing the “generate slop” button. Otherwise it’s not needed for normal, human emails.