“Even” or “odd” number isn’t as applicable as it sounds, though I do remember if you were an “s” year or not. It was iPhone 3G, 3G S, 4, 4S, 5, 5S, 6, 6S, 7, 8/X, XS, 11, and standard annual numbering from the 11 on. In early days you’d ask if you were an “S” year or not. Only since the 11 could you ask whether you were “even” or “odd” without it meaning upgrades every 4 years.
What’s changed is that Apple increments the model number every year instead of every two years, and you can probably blame Samsung for that a bit. Your friends probably still upgrade every 4 years like we almost always used to. I remember getting the 3G for the network improvement, the 3G S for the speed bump and video recording, the 4 for Siri, the 4S for a speed bump, … back then it was often the case that the non-S model was a new design with last year’s chip, and the S model was a speed bump with last year’s design.
If anything’s changed, it’s that they often spec bump the processor every year in minor ways, instead of every 2 years, and the increases are steadily more incremental than they used to be. If you ask me, it’s because Apple doesn’t have competition in the mobile phone CPU space yet. (Or ever, while Qualcomm holds so many patents…)
Also it’s more clear than ever before that the iPhone X, Max, Pro series are all ways to ask for more money for the exact same chip/product. Some years the Pro is significantly faster because it gets the new CPU first, but by the next year both Pro and non-Pro end up getting the same CPU for cost reasons. It’s telling that most years Apple doesn’t claim a speed increase between the Pro and non-Pro models. For example, if I recall correctly the only difference for the iPhone 13 was actually software. Likewise in this generation (16) it feels like the only difference between Pro and non-Pro is the SoC binning and the binning of the quality of the OLED in minor ways.