Yesterday, I tried to create a custom slot type (with custom argument). After hours of trying toe resolve, why every time bot complained about changing domain.yml, I found out, that custom argument provided in the domain file is not saved into domain file in the model folder.
After a day of digging into the internals of the Domain class, training functions and model creation, I gave up. Thankfully I got an idea to check the Slot class and found out the peculiar method: persistence_info. Shortly after (2 LOCs) everything worked like a charm (I am feeling really dumb right now -.-).
Thusly, I propose that it should be mentioned in the documentation, that it is also necessary to update the persistence_info method while creating a custom slot with a custom argument(s).
Have a nice day!