In my opinion, assassins is utterly unplayable. Everyone in diamond gear camps near shop and the ones who don't get eaten alive by anti knock-back kill aura script kiddies(proper term) and anti-knockback speed script kiddies
One of the biggest problems in assassinations for me personally is that people camp inside and near the shop and they don't leave this area, they seriously stay within 10 blocks of the shops at all times, talking about the players who have diamond gear of course. If your idea would get implemented it would give these campers an even bigger advantage, if the guy really isn't able to get out of the shop without dying, which is quite weird as he can leave the shop from every side, then he can always log out when out of combat and log back into the same map and he will spawn somewhere else probably far away from the shop where he was.Maybe "/i" makes you invisible after 60 seconds of being in a shop area without getting hit after that you can't interract with other players until you do /i in a shop area and on the ground (thus making you visible again)
While invisible you can spectate other players.