A "board" is another name for an internet forum, originating (as far as I know) in the long-ago days of online "bulletin board systems," which were commonly referred to as boards. It's not a term that gets much usage these days, and it will fade a little more later this month when Riot closes the League of Legends Boards.
"Starting March 9, 2020, we'll be closing down the League of Legends Boards," Riot said in the announcement. "We know some of you use them regularly, so we didn’t arrive at this decision lightly. However, after looking at how many of you were using Boards, it became clear that the popularity of the platform had decreased significantly. Both players and Rioters have migrated to other community discussion platforms such as Reddit and Discord, leaving Boards underpopulated."
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