I agree that on most rounds the prison trap would be inconsequential, but on the occasional round where someone triggers it, it'd just end up as annoying. It just doesn't have much reason to exist - I think the kid who made it just wanted to put that picture of his friend with the chili dog in more spaces.
I think it's over-trapped - most of the rooms around the tester are trapped, so innocents will just avoid them all and be forced to just stay in the main halls. There's no inherent risk to it for the traitors - there are multiple entrances, multiple exits, several traps, 0-risk c4 placement, and in the case of v7, even a knife spawner. I'm not going to add a map where we feel like we have to constantly police people trapping the traitor room - that's a sign of a traitor room that is too powerful. Map-specific rules suck - there's no easy way to communicate them to players and it just causes confusion and frustration.
I don't think there's much safety to being downstairs in v3 - you still risk getting drowned by going to the tester, c4 can be placed in the trap room which presents a serious risk to anyone in the tester room, and the cramped nature is perfect for chaos, with there being multiple escape routes for any active traitors. It's very frequently a high-traffic area, with far more deaths per square foot than any other area of the map.
The barn is a weird spot - I don't think making downstairs dangerous is going to funnel people there a ton. It's needlessly far out of the way and there isn't a ton there compared to the main street.
An organised duo of traitors (or innocents) would be able to easily grab the secret knife every round due to its lack of randomness. It's not a dealbreaker but it's pretty poorly implemented.
V3 isn't all that different compared to these later versions - they mostly just mess around with the balance of the underground. It's nowhere near as simple of a map as community pool or bowling - it's just what I view as the default version of the map, given I'm fairly sure it's the last one the original creator made.