Are you sure?
There’s also no note about Divine Steed’s duration being nerfed, so it should still be 5 seconds baseline. If its not, then report it as a bug and don’t assume its intentional.
Edit: I hopped on the PTR to check and do some testing. Bizarrely. Divine Steed is 3 seconds baseline for Holy and Prot, but its 4 seconds for Ret. Still assuming bug and its supposed to be 5 seconds baseline as before.
Edit 2: Prot is 3 seconds on Live, I had Warmode on. Oops.