Well they can still be serviced they just don't have dipsticks to check the level. Some have caps that can be removed to add fluid but some don't. The only way to service these transmissions is to use a flushing machine that hooks up the cooler line. Now this is a better way to service a transmission because it gets all the fluid out and many new transmissions either don't have screens at all or have screens that only need to be replaced when the tranny is rebuilt.
The problem is that really this is part of the never ending plan to get more people to come into the dearler for service work. Some dealers will try to say that only they have the right machine to service your car and will try to scam customers. I have only had one or two cars that I could not service with my shops machine and that was just because we did not yet have the adapter for that car. We ordered the new adapter and in a week we were able to service the car.
Another problem with the sealed transmissions is that not all of them come from the factory with synthetic. I am not sure which ones do and which ones don't but I know there are some that don't.
Just off the top of my head here is the list of cars that come with sealed transmission.
Most new BMW'* and Mercedes have sealed trannys and use synthetic fluid.
I think some Volkswagens and Audis do too.
Many new Izuzus have sealed trannys.
The chevy cavalier/pontiac firebird and some grand Ams have sealed trannys.
I am going back to work at my shop in a couple of weeks so then I will be able to look up the hour rating on synthetic tranny fluid. I think it is around a 1000 hours and I know that does not sound like much but it works out to a little over a 100,000 miles for most cars.
1989 LE Bonne 122,000 miles
Custom Open Element filter system, Strut Tower Brace from 2nd gen bonne
Light Up Pontiac symbol in grill, 16 wheels from newer Bonne/G, performance tires, performance ignition system, Upgraded brakes.