Can't say I'm surprised. It was circumventing the Guest Campaign system.
But I think they would be better off scrapping the Guest Campaign concept completely, and allowing people to assign themselves one campaign for leaderboards, and being able to enter any of the others freely.
What they have done by removing this functionality completely, combined with the fact that you can no longer group-queue into friends' campaigns (verification needed?), is that they have prevented numerous players from playing with their friends in Cyrodiil, which goes completely in opposition to the fact that they removed faction locks specifically to enable playing with friends, and also goes against the whole One Tamriel design ethos.
People used the PvP group finder? I didn't know if it actually worked!
So it Pug's you into someone else's group or??
Nah, to use the PvP group finder you need to already be in Cyrodiil, and It groups you with others in that campaign.
To use the group queue, you would form a group outside Cyrodiil, the group leader would queue for their Home or Guest campaign, and everyone in the group would be queued for that campaign too, regardless of what their own Home or Guest asignments were. ZOS removed/altered/broke the group queue a few weeks ago, which is when QueueForCampaign surfaced.
QueueForCampaign was an API function (so not available on consoles) which you could use to queue for any campaign, regardless of your own Home or Guest assignments, by using the Campaign ID of the campaign you wanted to enter.
Long ago we could change guest campaigns daily. No cost iirc. They changed that due to groups going into other campaigns and creating an imbalance. I recall a campaign where players that did not normally play there came in for the sole purpose of deposing the emperor.
This was in the early days of ESO when Cyrodiil was fairly active in multiple campaigns. It was probably due to activity like that and maybe other actions Zos noticed that they increased the amount of time that had to pass before one could change their guest campaign and iirc, Zos has added a cost to do so (need verification since I do not change campaigns often).
In other words, there may be behavior that Zos wishes to change, but of course it would be beneficial if Zos actually explained such things. I just did a quick search of the patch notes and nothing came up with the key words I used. Maybe I was unintentional. Not like Zos has not made mistakes before.