There was a thread with this topic being addressed by @ch3nky but people did not arrive at a consensus. I'm therefore reposting in the rules thread.
The ESL ruleset states that only one shield operator is allowed (see below):
All Operator are allowed (DLC allowed). In exception the new season pass operator are forbidden to use, till the admin staff announce this official.
Only one character using a shield allowed per team (this includes Blitz, Montagne, Blackbeard and a Recruit with a shield)
I think we should go ahead and update the rules. At the end of the day this is a competitive ladder and following ESL seems like the right choice. I can see why they have implemented it, and have to say i totally agree. With the introduction of Blackbeard something had to be done.