For starters, I 100% recommend people check out 
One-Shot Wonders, which is a big book of modular adventures.
It looks like the 
excerpted versions on D&D Beyond only go from levels 1 through 8, but the big blue hardcover book has a chapter of legendary adventures, all level 9 and up and two for level 19. And all of the adventures have notes on how to scale them way up to be an even harder threat.
ENWorld Publishing also sells a book called 
High Level Adventures which includes adventures from levels 10 to 16.
And Elderbrain sells 
Torrents of the Spellhoarder, a nautical campaign for levels 16 to 20. And all Elderbrain books are ridiculously supported with additional material.