While I love that Apple provides security updates for older iOS releases (The iOS 12.5.2 security upgrade from March[0] for eg) - Apple doesn't have any guarantees around it.
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.