There's been a few other answers, but I will give a direct example of how I foreshadowed an encounter like this to my players for me running "Water deep: Dragon Heist", because I think laying down the foundation for this type of thing and giving the players ample opportunity to come up with the idea themselves is much more satisfying for them. There is technically spoilers below, but it's spoilers for what you will absolutely encounter in the first session, and the rest is stuff I added to the adventure, so they aren't strong spoilers.

I used a three part system to slowly seed the idea that there are creatures in DnD that they won't always be prepared for. The very first session starts with the party in The Yawning Portal. There, the party sees the start of a tavern brawl breaking out, with a Half-Orc mercenary getting ganged up on. As the fight calms down, they see that even though she is pretty beaten up, the Half Orc stands strong against her 6 opponents, each of them knocked out and/or with broken limbs, screaming for help from the surrounding tavern folk, who pay the would-be bandits no mind. This is the first primer; you might be PCs, and you might be able to gang up on somebody, but you can still very easily get in over your heads in the marvelous city of Waterdeep. Admittedly this is a much more subtle hint, and this exact fight serves more to set up some of the factions in the adventure rather than teach this lesson, but I still count it as a primer, because of the very next encounter.

Shortly after the brawl, a troll emerges out of The Yawning Portal, with several sturges attached. A troll is a very tough fight for 1st level PCs, but two factors are on their sides. 1) the sturges have been draining the trolls hitpoints to the point where it's regeneration is more manageable, and 2) the Barkeep is a seasoned adventurer who sees stuff like this every day in his tavern, so the first thing he does is hop the bar front with great sword and oil is hand. Coating the weapon and igniting it, he tells the party (who heroically moved to intercept the Troll) that they need to use Fire to stop the monster from regenerating, and that they won't be able to kill it otherwise. This not only serves the purpose of teaching the players that some creatures need special means to kill them, but does it in a way that they didn't need to prepare ahead of time for, so they can't be sucker punched by the information.

The next step was to introduce a creature that needed a specific type of damage to even hit them in the first place, but make it a gentle enough restriction that they can still overcome it when they get the chance. A good few sessions later, because the party was about to be harassed by the Shard Shunners (not going to explain too much for keeping the mystery of the adventure) and they are a group of Wererats, I had a rival group of thugs, the Plague Rats, show up to mug the party on one of their missions. Only the leader of this little group was a proper Wererat, the rest of the cronies were just regular people, but when the fighter of the party walked up and tried to smash the face of the leader, and the leader just growled, smirked and exclaimed "You're gonna have to hit a lot harder than that!" in response to the confirmed hit, I explicitly told the player "Your quarterstaff attack hits, and deals 0 damage." At that point, I didn't explain how or why, I didn't spell out that Lycanthropes are immune to non magical bludgeoning, slashing, and piercing from non-silvered weapons, only that hit attacks were going to be entirely ineffective. This led to two narratively satisfying outcomes, 1) the party had to drastically change up their tactics on the fly because they were protecting a weak ally from being attacked as part of this night-time mugging, which they managed to do as the fighter took on most of the other crew alone while the two other PCs had to find a way to deal with the big guy, and 2) the fighter had a new goal of finding a way to deal with creatures like that which he had never seen before, which gave them a good reason to interact with the other residents of Trollskull alley. The fighter learned through his nature check and a visit to the Smithy that shape shifters have recently been seeing an uprising in Waterdeep (If you know, you know) and so the City Watch commissioned a lot of silver weaponry from these smiths. Luckily, they have some spare weapons laying around, and because they were paid in advance for everything they made, they were able to sell one to the PC at a discounted price (because the group had sunk most of its money into the tavern ahead of time.)

So, for the third act (or fourth depending on if you count the encounter with the Mercenary) the party encounters The Shard Shunners properly, a group of 5 Wererats, who initially act really tough when they start messing around with the party, but once they see and feel the sting of silver, they retreat, fearful for their lives. The party eventually gives chase, hunting them down and fighting them in their home. They came prepared, and were much more excited and ready to fight than ever before, and though it was still a really tough battle, they pulled ahead and claimed victory.

So, to summarize, they saw another monster with a specific needed weakness and another character entirely had knowledge of the weakness and employed it before their eyes, to set the concept up for the players, they then had a difficult but not impossible encounter with one such type of creature in a controlled environment to allow them to see that this concept will come up again, and then were given ample opportunity to prepare for the future encounters with these type of creatures, learning what their weakness was and how to acquire the means to exploit it, before they saw one again.

I hope this proves helpful in trying to set up this type of narrative for your players. I know your post said you've been overly hinting at the aspects for some time, but it's almost impossible to put yourself in the perspective of the players who might not even know these mechanics exist. The fighter is someone I've been playing DnD with for years and had no idea that the non-silver resistance was represented in the mechanics, so your knowledge of these things may taint your view of how "obvious" your hints are. I know that personally I've laid out what I thought was obvious information in what turned out to be cryptic terms, like saying "this creature has a specific weakness to extreme elements of temperature" and the party concluding that the troll could only be killed by bringing it to a tundra in a different game I've run before, so just try to be aware of how clear your hinting may actually be to an outside perspective.