I'm switching to 10 LTSC (nb they renamed LTSB) 1809 this weekend to get ahead of the curve. Supported until January 2029. 11 came out six years after 10, so decent odds this can tide me over until 12. Unfortunately they reduced their support commitment for the upcoming 10 LTSC 21H2 to five years from release, so the older version is the best you can do.

Also FWIW, Microsoft does kind of give you a choice to obtain 10 LTSC legitimately, it's just a pain in the ass since it's very deliberately not intended for consumers. Described here: https://community.spiceworks.com/topic/2167558-explicit-inst...

I actually did it (using CDW, total cost US$319.15), in part because I'm a goody-goody I guess and in part because even if you can trust the ISO you got is good by verifying its checksum, god knows what the tool you use to bypass the copyright enforcement mechanism does.

If anyone reading this comment is interested in buying it, you should probably also know that the Open License volume purchasing program that the linked units are being sold under is getting discontinued in January 2022, so might want to decide whether to bite the bullet soon.