Yep ... when I was in OK, you didn't need trailer plates. From no plates to a non-expiring plate (lifetime plate) is probably a good step since if you pulled the trailer out of state, the Police get suspicious. LOL.
In MI, there was one price for lifetime plates on trailers also. I had on on my deer camp bunk house.
It's pretty silly IMO. They are so strict about cars. So many regulations. You can be fined for having a brake light out or window tint too dark. There is a mandatory annual inspection of the operation of safety features on your car to ensure you are not a danger to others. Yet they don't give a damn about trailers. Give out permanent license plates, trailers never get inspected anyway. You don't even need to present your trailer for initial inspection. I have pulled & seen others pulling "home made" trailers (piece-mealed together from rusty scrap metal & old truck axles) with NO structural integrity. Could fall apart at any time. My cousin built a BBQ smoker trailer out of a 6000lb old propane tank, and as he was pulling it down the road the tongue broke off, and the trailer went on an excursion all its own. Mowed down a long section of cow fence and took out a light pole. I have seen a home made landscaping break in half above the axle when loaded with mulch going down the highway.
Here you need a registration number for your boat, and license plate for snowmobile and ATV, but it's a one time $25 fee and they don't expire.
Cars, Trucks, and Motorcycles, Trailers, even those owned by dealers expire.
Funny part is police cars have to have the sticker color of the year on theirs as well.
We can no longer have 2 sets of plates for a car though, that was nifty. You could put on your ego plates for the summer and standard plates in winter (or vice versa), now you gotta pick ego plate or standard plate.