Apple supports older devices for 5+, sometimes even damn near 10 years with feature and security updates. Regardless of if you agree with the feature changes, you should ALWAYS update to the latest iOS for security.
So it’s fine if apps only support current versions. This also allows them to adopt new tech which is great (Mlem is still one of the only third party apps that’s adopted Liquid Glass, along with randomly github, and that’s partly due to not worrying so much about older iOS)