I'm fine with not hinting every random encounter. I want to reward players who invest in feats and perception. What's the point otherwise if surprise can always be avoided by foreshadowing?
As I've said more than once upthread, foreshadowing doesn't always obviate surprise. Consider some outcomes after the DM shows that ankhegs are in the area by showing collapsed tunnels, molted chitin, and the partially-devoured corpse of an orc in a puddle of acrid bile: The players ignore it and carry on. The players examine the clues and draw the wrong conclusions due to inexperience, ignorance, or failed ability checks. The players try to avoid the potential problem by switching to a fast pace (and get a penalty to passive Perception). One or more characters has a flaw that makes him or her act hastily or fearfully and the player wants Inspiration. And so on.
Even if they
do figure out that ankhegs hunt in the area where they're walking, the ankhegs are
still trying to be stealthy and get to roll a Dexterity (Stealth) check against the PCs unless they have taken some steps to detect them (maybe through a ranger's Natural Explorer class feature, a druid turning into a mole or the like and trying to find them, or some kind of magical spell). Even if you're generous and grant advantage on their passive Perception (+5) because they're all alert to the presence of an anhkeg in particular, the dice may not go their way.
So let's put to rest the notion that foreshadowing always negates the chance of surprise, shall we? What it does do is create the opportunity to engage with the exploration pillar of the game, builds dramatic tension, and gives the players a fair chance to improve their odds against the fake difficulty of a random number generator.
When I DM, all my cards are on the table. Some are face up and others are face down. If the players want to get a chance to see what's under my face down cards, they have to do something proactively in the game to make that happen. They might try, might not try, might succeed, might succeed at a cost or with a setback or complication, or might fail. What I don't do is suddenly pull the card from up my sleeve and throw it on the table while shouting "Gotcha!" Because by my standards, that's not a fair or challenging game.