Blizzard announced beforehand they will be more stringent on realm management for SOD here:
While it is true the ingame Classic SOD transfer page uses the same client as Classic, it was never possible to purchase Seasonal transfers seeing SoM for example. I believe this needs additional ingame clarification as well.
The Classic transfer page on the site however does mention this:
Generally GM’s do not take feedback, but you can submit it through the ingame box or at the forums which you did.
For transfers in general not limited to only SOD realms there’s an additional stipulation that you potentially cannot transfer to full or locked realms as then players would just bypass the lock by paying which isn’t the intended option. They refused to take your money you see to allow this to happen.