But the reason I come down on the 'legitimate mechanic' side is because I've seen it happen in pen-and-paper sessions. [...] A group of low-level adventurers traipsing around the Underdark stumbles across a creature that - from appearances - we presume to be evil and VERY powerful. [The party] attempted to push it off the cliff on the first turn of combat, and rolled a natural 20, sending this incredibly powerful (and plot-important) NPC off the edge into the depths of the Underdark.
Was this plot important and very powerful character positioned
Right on the
edge of a
lethal bottomless pit, having been placed there
deliberately by your DM? Were they also no more than one size larger than your party members (for medium creatures; huge or above = no can shove) ? Did they also roll a contested check against the shoving party member, using their most advantageous ability score between Dex and Strength (one of which they were presumably very good with, unless they were a spellcaster and thus not scared of simple falling), and still fail?
Unless all of those things are true, than you were benefiting from over-reaching homebrew, and it's not a good example - fine for the sake of fun and spectacle, but if it was a legitimately important figure that your DM needed to use, then they would have been completely within rights to simply tell you that that's not how it works. If all of those things were true, then you DM was almost certainly setting the character up to be killed/shoved down that pit deliberately, regardless of what they report to their players.
