I'm working on having my group of 15th level norse-theme lads reach 20th level. At their current level, it is proving a little tougher than before, but by no means impossible, to put down good fights; a trio of Bulettes, for example, might not do a lot of damage, but they still provide entertainment. Lyn Armaal, the Cloud Giant castle from SKT, has been a perfect challenge for them - tough, with really brutal combats, but achievable.
Bounded Accuracy means that you can use monsters of the CR 2-5 range happily as 'adds' or 'chaff', depending on terminology; the players enjoy having small threats to burn AoEs on. And a CR 9 Cloud Giant hitting the Monk or Paladin twice in a turn - happens easily with +12 - results in some very appreciable damage being dealt. (The Paladin is especially vulnerable to their new Fling ability, doing 6d8 damage and knocking him prone and separated 60' from the party.) However, some player abilities become really hard to counter; Cloud Giants have absolutely no answer to a Rogue with stealth +15, a cloak of Invisibility, and a Broom of Flying. He has taken no damage at all in the castle, except on the one turn when he rolled a 1 for his stealth (houserule in effect for auto-fails on 1).
So as far as adventures go, it is not impossible to put down fights that the players will enjoy. However, I think that you will definitely never TPK any party above level 12, in practice, and will probably struggle to really challenge them except with a LOT of bad guys. The difficulty is in finding bad guys that can reasonably exist in such numbers and at such power levels. As always, heading to the Planes is probably your best bet; a sudden infestation of 80 Frost Giants in your Material Plane setting rather begs the question of why they didn't conquer the whole kingdom back when the PCs were level 1!