There must be something going on in Bangkok when the airport added turnstiles and warning to remind travelers to get taxis from the arrivals level.
For years, many of us have taken the cab from the departures that allows you to pick and choose the cab you want and save few baths on the airport fee. Despite all the efforts by the airport you can still do this.
Once the turnstiles were first introduced, there were briefly fewer taxis loitering waiting for pick ups but this has changed. The situation is back to almost to the previous normal level.
You don’t have to worry about the turnstiles either. You can easily get bag through or get a driver to help you lifting.
Also, there are guards/traffic police, but their job is not to prevent folks crossing and taking a cab and they have never said anything to me.
I would only take a cab from the departures levels IF you know what your destination is and/or you have a local number just in case you need to get directions to the driver.
The drivers at the departures level are happy to get a client so that they don’t have to wait hours in the line to get one from the arrivals or to drive back to the city empty.
At the arrivals level, you often have to wait in line AND you are not able to pick and choose the car/driver that you would prefer.