Highlander - Yes
6v6 - No
4v4 - Depends
In Highlander numbers are less important so you can utilize a death pit as a new strategic area. An example of deathpits used right in a map would be the side path that leads to a great sniping point, a health pack and a flank on Keikoku. It has high risk because you can instantly be killed by a pyro/well placed rocket, but is a fair tradeoff for what you get.
Another example of deathpits done right is Upward last, it creates a remix of the usual "no man's land" scenario you see in cp maps, where each side of the divide has a great advantage over the middle, and a neutral advantage over the other side, but with this being a death pit, it requires players to either pass through a small choke or jump over it. (losing heath in the process).
In 6v6 every player counts. Usually if you lose a Scout/Roamer you really can't fight an equivalent team without an Uber advantage. So minimizing the amount of random instakills that can happen is a great way to get people to like your map. Imagine Sunshine where the whole edge of the 2nd point is a death pit, that doesn't sound very good. It would force players to constantly stand in the lighthouse, susceptible to spam, and if they did want to walk up the stairs there would be nothing stopping a Soldier from instantly knocking them away, which would be a big problem for Medics, since Roamers constantly try to get them off guard.
In 4v4 it's a weird situation. While players definitely do still count, because of the high amount of class disparity and different play styles of different teams, a good team of Scout/Pyro could totally outdo a Scout/Soldier/Demo if it were to come down to it, which doesn't mean spam death pits all over the map, it just means you're not barred from using them as with a 6v6 map, because the level of DM is so high and the amount of serious tactics outside of your team composition are limited death pits would do lovely to spice up gameplay. Although through saying this I don't think any map played in 4v4 has a death pit, so maybe keep that in mind.