No, you can't go back to an older iOS after updating. There's often a small "grace period" after a new iOS release when you can downgrade, but it's only for a week or so.I haven’t used or seen IOS 18.1 at all means that I have no idea how the menus have changed, just that I know they won’t work.
It's an interesting article and it's such a shame that Apple have removed the option to apply an audiogram to all of the AirPod range including the Max.
It was the FDA in the US that approved the use of the Airpod Pro 2s as OTC hearing aids and, at a guess, Apple were forced to remove the Audiogram option from the other products as part of the approval rules.
As far as I know it's not possible to go back to an older IOS after the 18.1 update but it wouldn't stop me from trying if I had an old backup available. As I said earlier, I was able to set the AirPods up again on my 2018 iPad running 17.7.1. Am old iPhone 12 would do the job.
So many people just let their phones update whenever Apple brings something new out. I prefer to let the dust settle and let everybody else wrestle with the bug and inevitable glitches. As a general rule I leave well alone until .4 or .5 is released whether it's on my phone or my desktop.
Now I am constrained by the wait for the MHRA in the UK to approve them before I can update.
If you did apply your audiogram to your headphone/earphone before updating to 18.1, those settings do keep working on 18.1. But you can no longer modify the settings/apply another audiogram.
Also, if you have multiple iDevices, and one of them is still on iOS 18.0 or earlier, you should be able to apply an audiogram on that device, and the setting will carry over when you use that earphone with your other devices. At least, it does on the AirPods Pro 2. I think it should be the same for the others, but who knows?