To my knowledge...these spots are illegal to barricade on:

zm_lila_panic_v2 - money crate room
tx_highschool - red room
zm_ruinedtown - glitching the doors in the bathroom stalls
fiddlersgreen - glitching the elevator door

There will always be a spot that people will get pissed about. Problem is you can get in with team work or multiple entrances. The following are the only ones I know of. The money crate argument is a lie. In tx_highschool in the stage room, I've seen people barricade with the money crates to the entrance but leave the vent open. I've also seen zombies lure humans glitching the money crate from the vent entrance under the stage.... so you can get in with team work.

someone had the nerve to mention the teleport entrance in fourcorners to the office upstairs is illegal because zombies can easily get stuck in vending machines and couches. NOT TRUE. A zombie can easily press E more than once and fuck up that cade tagging you. If it's difficult for you to break out... let someone else do it. Mentioning spots that others can easily break is more of a personal preference as to either you suck at breaking cades or just want to give people a hard time. Do not ruin the already limited spots in ZM.