Exciting news for anyone interested in set contents: instructions for 30 sets from 2018-2019 have recently become publicly available. They're not accessible through the official website, but you can view/download them with direct links, which I've added to the set pages and setlist items.
Official Instructions Newly Online, 2020-03-13 Even better: quite a few of the PDFs were created with the part-numbers in a separate text-layer, which means the numbers can be copied and pasted with the text-tool in PDF readers. I've taken advantage of this to scrape part numbers right out of the PDFs and add them to the inventories! I'll be verifying the sets contents by hand over the next little while, but I've got pretty good confidence in their accuracy.
Some 2020 sets have been released with this feature too: not all of them, but some new Soccer, Spirit,1-2-3, and Zoo sets are fully (99%) inventoried from this method. I'll be keeping my eye out for more. Hope they keep it up!
2020 Soccer, Spirit,1-2-3, Zoo sets with instructions