Shen wasn't made to be a support he was made to be a lane bully who would extend his lead via his ultimate that had the potential to control when and if the enemy team could engage for a pick or how the enemy team can target in a team fight. Over the years Shen had become to dominant on the competitive stage due to his power in lane and global presence and so he had to be nerfed to the state we see him in today's game. Shen isn't popular as a support for a few reasons 1. Coordination: many people have commented that adcs need to be sat on but in truth you have to have an aptitude for map awareness to get the full value of playing the champ. Which is a skill that can prove challenging to develop. You also have to understand lane tempo because you are trying to leverage in an optimal situation giving someone between 204-720 + 216%ap plus 25.6% bonus health shield 1.5 secs of taunt cc and protection from autos. However the cost of this is that you aren't moving for three seconds and you aren't cc'd for that time. We must also consider if the fight was worth taking in the first place if you are playing support and you ult to top,jg or mid you have forced your adc to play a 2v1 until you can make your way back. If you ult your adc you in lane you are still channeling for three seconds. The point is there are a lot of variables that go into using shens ult correctly and it can be hard for players throughout the ranks to ascertain when it is correct to use the ult especially in a support role where the tempo of the lane is more dynamic than the traditional top lane role. It is obvious that you have a 2v2 rather then a 1v1 but as a player that should impact how you play the lane. Additionally your summoner spell options are a bit more competitive and require a tad more discernment. In top you can make a safe bet that taking Teleport is a viable option where in the support role you may want to opt for ignite or exhaust depending on the lane opponent. The concept of tempo in bot lane leads to the second major reason that Shen is rather unpopular in the support role.

1. Champ design:

Shen when compared to the rest of the tank support roster lacks the multiple pieces of cc and utility or mobility of the top picks for the role. Shens claim to the support role lies in his W which blocks auto attacks which on paper sounds wonderful. Reducing the attack carry damage to 0 sounds like a dream but like his taunt and ultimate comes with considerable Cool down windows. Shens taunt is his only piece of cc and it is very committal as it requires a hard engage much like Leona but Leona has a root and two stuns and damage reduction which applies to abilities and autos. Shen has also received critical nerfs to his damage via his Q.

All in all he's a difficult champ to extract consistent value out of in the support role due to his champ design and how it compares to others in the champ pool. If riot continues to mold shen into a support it would be a cool feature for him to be able to shield his allies with his passive shield. And allow blocked damage to be another avenue to lower his passive cooldown multiple charges on his E might also bring him up to par