The thing about the beta UU matches is that it also might be a different environment than OU. Decidueye's niche for OU teams is based partly on him being a good answer to things that are more prevalent in OU, like Ground Types, Sand, bulky/passive waters, and breaking hazard stacking stall teams. The fact that he performs poorly in a lower metagame (that hasn't even developed itself either) does not 100% indicate he will not have a worthwhile niche in OU.
I also think people are thinking about Spirit Shackle wrong. Spirit Shackle's offering isn't the fact that Decidueye can trap things he checks, but he can trap his own checks and make them very easy to counterplay since Decidueye has to leave the field before the opponent can choose to leave with anything besides their own pivot move. For a somewhat predictable/common example, the opponent can't bring in Heatran to answer Decidueye because if he goes for Spirit Shackle, Heatran's stuck in until Decidueye leaves, leaving his friend Dugtrio to watch the house. Similar case with, say, Skarmory or Celesteela and going to Magnezone/Alola Golem. Even barring other trappers, the momentum of forcing your opponent to wait until you bring in a check gives a huge amount of momentum for something like a set up sweeper (Z-Rain Dance Manaphy in front of Heatran, Sub attackers in front of something like Amoongus), and the main defensive mon Decidueye can't trap, Chansey, won't come in on him anyway since he can blow away Hazards, is immune to Seismic Toss, and can either SubPass, SubToxic, or just plain Toxic her. SubPass is deadly if passed to something like Mega Gyarados to set up (imagine giving it to a Disguised Mimikyu, 2 turns without damage to reach +4 unhindered is nuts), and Toxic means Chansey can't even stay in to PP Stall Decidueye out, again forcing her to leave and letting you take momemntum with a Double switch, U-Turn, Baton Pass, or more Spirit Shackle mindgames.
This thing will have very specific offerings, but also looks the type where if you need what it offers, it can pay off VERY well with role compression and/or its particular niche.
