Every developer develops for the newest devices first and foremost. Across the board.

There is nothing intrinsic about a smaller manufacturing process that transfers into software, unless they’ve secretly added new instructions to the set. What it does mean is that the new chips should be more power efficient, which means in turn that the same software on new hardware should already translate into battery life gains.

Orrrrr what it means is that the software can take advantage of the more efficient hardware and push harder while maintaining similar battery life. Which is what most manufacturers typically do.

What we see instead is software “tuned” for the new version to minimize gains on one side and suggest to existing customers that they need an upgrade.

Welcome to every piece of technology ever made. If you want the latest and greatest software performance, you need to buy the latest and greatest hardware.

Being less optimized doesn't mean you need to upgrade. You may need to charge a little more often but that's the trade off for running new software on older hardware. You can still run the latest software and don't need to upgrade.

Apple supports iPhones for a minimum of 6-7 years. The iPhone XR from 2018 is supported for iOS 17, 6 years and 6 major OS updates. It will keep getting iOS patches until they stop doing OS updates on iOS 17 sometime late next year. They support security updates for even longer. They just released a security update for the iPhone 5s, released in 2013, in January this year. Almost 10 years later.

Forced obsolescence is MUCH more common with Android phones. An $1800 Pixel Fold released in June this year, 2023, is only going to get Android updates until June 2026. 3 years for a nearly $2000 device. You will HAVE to buy a new phone if you want new software after that. Which is absolutely ridiculous for Google, who develop Android ffs. An iPhone 12 Pro Max from 2020 will be supported longer than that.

A Galaxy fold, another nearly $2000 device, will only be supported for 4 years before you will HAVE to buy a new phone to get a new version of Android.

Most other Android phone makers, especially the budget ones, only give 1-2 years of support, if that.

And again, OP's case is anecdotal. I have a base iPhone 12 from the same year, and have been running the iOS 17 betas since the first public one and have seen no drops in battery life. The first beta had a little worse battery life but the following ones fixed it a bit. The last public beta from a little while back is the exact same as the release version. You don't even get a separate update for the release version if you are still on the beta profile, and if you want to leave the beta once you are on the RC, you just turn off beta updates and you will only get the release versions from then on. If OP is suddenly seeing a huge battery life difference, it isn't the version they are on because they have been on it longer than they have been seeing the issue. There are things they could look at, which others in the thread have given, and they could look at their battery health. That's a 3 year old battery and it may just need a replacement depending on how hard they have driven it over the last 3 years. Batteries are consumable parts and need replaced.