iOS 15 is now available for all iPhones back to iPhone 6s, iPhone 6s Plus, and iPhone SE first generation, all released in 2016, and all iPhones since. (also including iPod Touch 7th generation, from 2019). On older iPhones, some features are not available, because the device doesn’t have the necessary hardware.
My recommendation for iOS 15 is to go ahead and upgrade. It’s stable and brings some nice new features.
As always, I highly recommend double-backups.(Triple-backups, if you really want to count.) The simplest way to achieve this is to backup to iCloud and also to a Mac using the built-in backup capability. Plug your phone into your Mac and it will backup in an hour or two. Here are step-by-step instructions: