old iPhone errors, such as "cannot verify server identity" or
An iPhone 5s /w iOS 11 (or newer) is the technical minimum requirement for VDO.Ninja, as Apple's webKit lacks the proper webRTC support in older iOS versions. Older iPhones/iPads can't update to newer iOS versions, so they are out of luck. iPhone 3G, 3Gs, 4, 4s, and iPhone 5 are not supported as a result, and many older iPads are also not supported.
I'd strongly recommend at least iOS 16 be used either way, as older versions of iOS have serious limitations and bugs. Starting with iOS 13 things started to be at least usable though, so an iPhone 6s is the recommended minimum, but don't expect a bug-free experience.
If using the native iOS app of VDO.Ninja, while I used to support iOS 11 with it, newer releases of the native app may require a newer iOS version. If using an iPhone 6 for example then, while it may not work with the native app, it might still at least work with the Safari web-app version.
Older Android phones tend to work usually though, as I think Google added support for WebRTC earlier on than Apple did, but you might be limited in terms of maximum quality or face heat issues on such old phones.