As part of the commander rules, commander decks don’t have a sideboard. Decks shouldn’t be labeled as ILLEGAL just because of a card in their sideboard. Please change the Moxfield commander deck legality rule to “no banned or illegal cards IN MAIN DECK”. I use my sideboard for playtest cards, or for new cards which have not been released yet. For example, my Rabbit commander deck. In the main deck I have a placeholder rabbit creature card tagged as “swap for Champion of the Hareish”. Then I have Champion of the Hareish in my sideboard(a playtest card, illegal in commander). This allows for the main deck to remain legal, while still giving the option to use playtest cards. I also utilize the sideboard for new additions which have not been released yet. A deck shouldn’t be illegal just because a soon to be released card is in the sideboard, waiting to be swapped in as an upgrade.
Technically speaking, there is at least one case where a sideboard is considered a valid part of a commander deck- companion cards like [[Kaheera, the Orphanguard]]. These may start out of play in Commander, and would need to be listed in the sideboard.
Maybe the solution here is for moxfield to differentiate why a deck is marked as illegal, so people doing fun rule zero things aren’t lumped together with someone who intends to cheat.