If I have 5 Pokemon on my Bench and Lunatone with "New Moon" Ability and Solrock are in play, what happens if my opponent plays Collapsed Stadium? Would I have to discard any Benched Pokemon or does "New Moon" prevent that?
The effect of Collapsed Stadium is being done to the player's bench size, not to the Pokemon on the bench; so New Moon would not protect them and you would have to discard Benched Pokemon.
Source: TPCi Rules Team (2023-09-07)

Relevant cards

Note: All applicable cards may not be listed below.
SV3_92
SV3_92
SWSH9_137
SWSH9_137