Has an MLS game ever ended 8-1?
Exactly once. On June 3, 1998, LA Galaxy beat FC Dallas 8-1, and no MLS game before or since has ended that way. When it happened it was a scorigami, the first 8-1 in league history.
That makes 8-1 the 31st most common final score out of 34 that have ever occurred in MLS. One point away, 9-1 has happened never and 8-2 has happened never.