Ever is an interesting word. From its earliest appearances, about the year 1000, it bore two related senses:
- At all times, in all cases, on all occasions
- At any time, in any case, on any occasion
The all sense, however, began to wane after the 17th century. It continued in literary use into the early 20th century, and it survives in such combinations as evergreen, ever-changing, forever and ever. But that sense is no longer 'productive' of new combinations, except when a user wants to create an archaic or 'elevated' feel. (However, the sense does survive indirectly in every, an adjectival form which derives from an Old English form equivalent to ever each.)
Today only the any sense is used in ordinary discourse.
So a good test for acceptability of ever is to substitute the phrase at any time. This one is fine:
OKThose are the slimmest thighs I have seen at any time.
But this one doesn't make any sense:
∗Those are the slim thighs I have seen at any time.
It does work, however, in questions, negations and conditions:
OKHave you at any time noticed those slim thighs?
OKI have not at any time noticed those slim thighs.
OKIf I had at any time noticed those slim thighs I have forgotten them.