That being said, the reason they delete instead of move is that, long-term, they're planning on having us be able to create our own custom boards.
Which begs the question, why is locking not the go-to way to address content rather than full-on deletion?
Moreover, Riot has two completely non-compatible design ideas here. The current forum is a forum. The eventual goal in like 6 months is a decentralized system, but many of the moderation policies treat this centralized forum as if none of the boards interact. This feels bad; even if the eventual goal is to transition the platform's functionality, it should really be important to have the moderators treating the current system properly.
Even more interestingly (to me), one distinct moderation policy completely disagrees with the future plans for the platform (decentralized). Why is it that threads are deleted for being cross-posted? If a thread belongs in a bunch of different Dominion boards (to use your example) and they are all entirely separate from each other, it should be fine to post that thread in all of the Dominion boards.
These things just don't agree with each other, and within the same system (moderation). It's a bit frustrating.