In the iOS world, not generally so. Things generally get faster and more optimized (cruft gets removed) in the iOS world (barring some anomalies like iOS 13, but it's gotten back on track).
iOS 7, 11, and 13 are definitely slower than their respective immediate predecessors. iOS 9 and 12 are definitely the opposite.
But while it's hard to prove definitively—largely because highly-out-of-date iOS devices are so rare in the wild—I would posit that even though 12 was faster than 11, 9 and 10 were both faster than 12. That's definitely what I saw in a brief iOS 9 experience I mentioned elsewhere in this thread.
It's the old frog-in-boiling-water analogy. If it happens slowly, you don't notice.
In the iOS world, not generally so. Things generally get faster and more optimized (cruft gets removed) in the iOS world (barring some anomalies like iOS 13, but it's gotten back on track).