Previously it was possible to manually set the aspect ratio of the Path of Exile client to be very short and wide, allowing you to see an unintended amount of the level at once. A limit needed to be set. Because of server serialisation distances, this maximum aspect ratio is currently set to 21:9. This is the widest that we can currently go before we start hitting the cutoff where entities and effects vanish. We are looking at being able to increase this in the future.This note was accidentally missed from the patch notes. We're very sorry for that oversight.

Previously it was possible to manually set the aspect ratio of the Path of Exile client to be very short and wide, allowing you to see an unintended amount of the level at once. A limit needed to be set. Because of server serialisation distances, this maximum aspect ratio is currently set to 21:9. This is the widest that we can currently go before we start hitting the cutoff where entities and effects vanish. We are looking at being able to increase this in the future.This note was accidentally missed from the patch notes. We're very sorry for that oversight.

What the client currently supports is what's intended.
I'm locking this thread as it is no longer a bug report.
Feel free to continue the discussion in the Suggestion & Feedback forum section.Edit: For clarity, that section can be found here:
https://www.pathofexile.com/forum/view-forum/feedbackThere is also an existing thread for this, here:
https://www.pathofexile.com/forum/view-thread/3294819Note: all the feedback from this thread and the other threads has been considered.
Originally, the way the resolution was handled on league launch wasn't ideal, a small change was made. Spoiler "Adjusted the maximum supported aspect ratio to 12:5 (slightly larger than before, so that players that were very close to 21:9 don't get black bars)."
https://www.pathofexile.com/forum/view-thread/3296830 I apologise if my initial reply was dismissive, that was not my intention.