UPDATE - The dataminers behind this discovery have admitted to a mistake. It turns out they were looking at source code for Super Mario Party and Mario + Rabbids Kingdom Battle at the same time, and got the code between the two games mixed up. The DLC mentions are for Mario + Rabbids, but the unused board does indeed come from Super Mario Party. You can find the original story below.
Many thought DLC for Super Mario Party was a lock, but nothing has been announced since the game's launch. It seems like the chance of DLC now is slimmer than ever, but dataminers have found evidence that points to DLC being in Nintendo's plans at some point during development.
The game's code makes mention of both ‘SeasonPass’ and ‘DLC1,’ which are hard to read as anything other than post-launch content plans. Why these never came to fruition is unknown, but it turns out Nintendo had some other plans that were scrapped as well.
Among the source code was data for an unused city themed board. This could have been a test board at some point, or a planned board that never made it past the initial concept stage.