The closest guarantee you get from Apple is that "iPhones on the latest iOS release are supported". The latest iOS list of supported devices is at [1], and Wikipedia[2] also considers that as the "list of supported devices", because Apple refuses to provide clear guarantees.
The Apple guarantees are on service[3], and only seem to apply for Hardware issues[4]. If someone has a better citation for iPhone 5s being officially under security support, I'd love to update https://endoflife.date/iphone accordingly.
[0]: https://support.apple.com/en-us/HT212257
[1]: https://support.apple.com/en-in/guide/iphone/iphe3fa5df43/io...
[2]: https://en.wikipedia.org/wiki/List_of_iOS_and_iPadOS_devices...
[3]: https://support.apple.com/en-gb/HT201624
[4]: https://apple.stackexchange.com/questions/370530/is-ios-12-s...
But I do care about security updates.
I would say that's the one place where Android got it right, you can keep having updated system apps for a long time after the OS itself is abandoned (which is shamefuly quickly).