Best supported result
Key to the Side-Door
{2}, {T}: Target creature can't be blocked this turn. {1}, {T}, Discard a legendary card with the same name as a legendary permanent you control: Draw two cards.
The Key offers repeatable unblockability and a way to turn a spare copy of a legend you already control into two cards. Those are specific needs, not generic artifact value. A deck considering it should have a useful attacker or enough duplicate legends to activate it reliably. The observed lists establish adoption; they do not by themselves prove which ability decided a match.
Tournament evidence: Key to the Side-Door appeared in 5 Modern decks across 3 separate events, with 10 total registered copies. 5 decks used it in the main deck and 0 used it in the sideboard. Example: Scorpion100, finishing 26 in Modern Challenge 32 on 2026-08-25.

