This is a community mostly about the Apple third-party developer program and related membership questions, and with some app-related development questions, and posters here are other users and other third-party developers. Not folks that work for Apple.
About your iPhone, an iPhone 7 is from 2016, with an A10 processor, and with 2 gigabytes of memory. In comparison, the iPhone 14 models have an A15 or A16 processor with 6 GB. As with most software over time, iOS is getting larger and more complex and with increased requirements.
Apple tends to support an iPhone on current iOS versions for five to seven years, and iOS updates typically arrive yearly. After that, there will usually be several more years where third-party apps remain available. After that, Apple patch support and third-party app support drops off. Right now, support is dropping off for iPhone 6 and earlier, as those are limited to iOS 12. iPhone 6s and iPhone 7 can run iOS 15, and most (nearly all) third-party apps will support iOS 15. Which means you will probably have a few more years where apps are available. Some of the larger app developers presently support the three most recent iOS versions.
For third-party developers, maintaining support for older versions has costs. It’s not free. Code supporting multiple versions becomes more complex, and new features (features which can convince users to buy the app) become harder to add, and app updates become harder, and testing requirements increases. And this added effort for those that are not updating and/or not spending on their hardware, and older hardware tends to be slower and more limited which means folks are less happy about (lower) performance and (missing) features and more work goes into keeping the app performance adequate on older hardware.
TL;DR: your iPhone is getting old. App support will likely be available for a few more years. After that, probably time for a newer (used, refurb) or new iPhone.
