My wife has a first-generation iPad Air running the latest iOS version it can possibly run (12.5.1). There is no updating it any further. She has backed it up using a 2017 MacBook Pro running macOS Catalina 10.15.4. She purchased a new 8th generation iPad. It came with iOS 14.4. She set up the iPad and then tried to restore from her backup. Finder offered her the option to restore from her existing iPad backup. She clicked that and clicked continue. It then prompted her to agreed to the "iPod" terms of service (which seems weird—it's an iPad, not an iPod). When she clicked "Agree," the iPad ejected from her computer and did ... nothing. She and I both have tried this multiple times with the same result. Nothing she or I do/try makes it restore from her 12.5.1 backup.
She contacted Apple tech support. They told her she cannot restore a backup from an older iOS version to a device with a newer iOS version. This is, of course, utter rubbish. It's actually the opposite—you can't restore a backup from a newer iOS version to a device with an older iOS version. So she's stuck with no way to restore her settings, passwords, documents, and, most importantly, game status (levels, accomplishments, etc.) to her new iPad, making her want to just return the new iPad and keep using her iPad Air until it really dies.
I know this can't actually be our fate. I've restored numerous old-iPhone backups to new iPhones in the past decade without a single problem, ever. It shouldn't be any different with iPads. Why can't we do this? How do we do this?