Alright, just compiled some really comprehensive updated lists of what each iOS device A12 and below are missing (the list includes all features missing, even from previous versions of iOS and the version they debuted in). I already posted what the 6S and A9X iPad Pros were missing, but I'll put them down again just so that
we have everything in one place. If anyone else knows of a feature missing on any device that I forgot, let me know so that I can update the list. I apologize if the formatting is inconsistent and the lists are all over the place, and for it being really long.
iPhones:
6S/SE1:
HEVC/HEIF encoding (iOS 11)
reading NFC tags (only QR codes) (iOS 11)
FaceTime effects (iOS 12)
Spatial audio (iOS 14)
Audio Sharing (iOS 13)
ARKit 3 and 4 (FaceTime eye correction included) (ARKit 3 in iOS 13, 4 in iOS 14)
CarKey (iOS 13)
Back tap (iOS 14)
On-device dictation (iOS 14)
Mirroring selfies (iOS 14)
Exposure lock (iOS 14)
4K content on YouTube (iOS 14)
1080p FaceTime (iOS 14)
7:
Audio Sharing (iOS 13)
ARKit 3 and 4 (FaceTime eye correction included) (ARKit 3 released with iOS 13, 4 in iOS 14)
CarKey (iOS 13)
Back tap (iOS 14)
On-device dictation (iOS 14)
Mirroring selfies (iOS 14)
Exposure lock (iOS 14)
4K content on YouTube (iOS 14)
8/X:
ARKit 3 and 4 (FaceTime eye correction included as it uses ARKit)
CarKey (iOS 13)
On-device dictation (iOS 14)
Mirroring selfies (iOS 14)
Exposure lock (iOS 14)
4K content on YouTube (iOS 14)
XR/XS:
Exposure lock (iOS 14)
iPads:
Air 2/Mini 4:
ARKit (starting from iOS 11)
FaceTime eye correction (iOS 14)
HEVC/HEIF encoding (iOS 11)
HEVC/HEIF decoding (iOS 11)
Hey Siri (iOS 9)
Facetime effects (iOS 12)
Messages effects (iOS 12)
Memoji stickers (iPadOS 13)
Audio sharing (iPadOS 13)
Spatial audio (iPadOS 14)
On-device keyboard dictation (iPadOS 14)
Apple pencil features (iPadOS 14)
Multitasking with three apps active at once (iOS 11)
Sidecar (iPadOS 13)
4K content on YouTube (iPadOS 14)
1080p FaceTime (iPadOS 14)
A9 iPad:
HEIF/HEVC encoding (iOS 11)
facetime effects (iOS 12)
ARKit 3/4 (starting in iPadOS 14)
FaceTime eye correction (iPadOS 14)
Hey Siri (iOS 10)
apple pencil features (iPadOS 14)
spatial audio (iPadOS 14)
Multitasking with three apps active at once (iOS 11)
On-device keyboard dictation (iPadOS 14)
Sidecar (iPadOS 13)
1080p FaceTime (iPadOS 14)
4K content on YouTube (iPadOS 14)
A9X iPad Pro:
HEVC/HEIF encoding (iOS 11)
Facetime effects (iOS 12)
ARKit 3/4 (iPadOS 13 and 14)
FaceTime eye correction (iPadOS 14)
Audio sharing (iPadOS 13)
Hey Siri (only supported on 9.7, iOS 9)
Spatial audio (iPadOS 14)
On-device keyboard dictation (iPadOS 14)
Multitasking with three apps active at once (iOS 11)
4K content on YouTube (iPadOS 14)
1080p FaceTime (iPadOS 14)
A10 iPad (both 6th and 7th gen):
ARKit 3/4 (starting in iPadOS 13)
FaceTime eye correction (iPadOS 14)
Multitasking with three apps active at once (missing since iOS 11 for iPad 6, supported on iPad 7 due to 3GB RAM)
On-device keyboard dictation (iPadOS 14)
4K YouTube (iPadOS 14)
1080p FaceTime (iPadOS 14)
A10X iPad Pro:
ARKit 3/4 (starting in iOS 13)
FaceTime eye correction (iPadOS 14)
Spatial audio (iPadOS 14)
4K content on YouTube (iPadOS 14)