First, you need evidence of the 5 man limit? You could just try to que with 6 people and see what happens. There’s your obvious evidence. But…
Here is something from 2012 in regards to blizzard’s stated stance on premades:
https://www.bluetracker.gg/wow/topic/us-en/7199643338-battleground-queues-and-addons/
Nov. 30, 2012, 7:33 p.m.
In patch 5.1, we resolved some issues regarding how Addons interact with the Battleground queue system. It is no longer possible to automate queuing for standard or random Battlegrounds in groups larger than 5. We decided to make this change in order to improve the experience for all players who are queueing into random groups normally and expecting to face similarly random groups, only to find themselves crushed by a full team of coordinated opponents.
We understand that players want to enjoy coordinated team play, and we invite those players to queue into Rated Battlegrounds or play Wargames to have that experience. Players can continue to use Addons to queue for Rated Battlegrounds as well.
Also this from 2012:
https://www.engadget.com/2012-12-05-blizzard-takes-a-stand-against-pre-mades-battleground-groups.html?guccounter=1
If you’re waiting for a blue post archive, unfortunately when they migrated and changed the forum, a lot of posts were lost. But there are references to old posts about the issue.
This post by Holycow references a series of tweets made by holinka in regards to 6+ man premades. Unfortunately he regularly deletes his tweets but you can find them in an archive if you need to go beyond just testing a 6+ que in the game you play. ( Epic premades are toxic and totally violate the social contract and terms of service - #28 by Holycow-uldaman )
The above is showing how they were considering reopening it for Ashran but they quickly realized it was a bad idea and reversed it.
As you can see, their public stance on 6+ man premades was made over a decade ago. They do not support it. They even had to actively break addons that were allowing groups to bypass the 5 man restriction.
Welcome to wow. No, you are not supposed to go beyond 5 player premades in randoms. They want you to only do that in Rated bg’s.
The rules are there and have been for quite a while. The issue is not having devs that care enough about pvp to enforce their own rules. And the issue of players thinking they’re allowed to break the rules because “it’s an mmo bruh. just let us stomp you, bruh!”
NOTE: They have worked to crush the premade issue in classic eras of the game.