Sorry to hear that, I understand. I never update iOS and I have several iOS versions. My iPad Air 5 (M1) on iPadOS 15 cannot access some notes created on my 11th-gen iPad (A16) on iPadOS 18, I just accept it. As you can see, I never update anything.
I think it’s just part of the considerations before updating. Some compatibility issues are eventually inevitable (I NEVER update anything, my iPod Touch 5G runs iOS 6, my iPhone Xʀ runs iOS 12, my iPhone 16 Plus runs iOS 18, I have well over a decade of experience managing iOS incompatibility, but I prefer it over having a garbage device because updates kill them), but I just accept them and tolerate them.
I understand that what I do isn’t always possible for everyone, but you couldn’t access that Numbers document elsewhere? Updating was absolutely required and there was no alternative? I ask because maybe if this happens again to you you can try to figure out a solution instead of updating. I’d always advise everyone to stay behind.
Just for you to see that this happens to me, too, I just accept it and access the note elsewhere:
View attachment 2638226
Screenshot from my iPad Air 5 (M1), running iPadOS 15.