Yea, i've no idea about the coding aspect of it. But i really can't imagine that having 2-10 various icon options would take up very much space, would it?
I do get what you are saying about brand awareness. But i don't think we are talking about a drastic difference (like a green version of iBooks for a Facebook app). The developer is supplying the choices. Take a look at some of the options that Apollo gives you... they are all pretty identical (slight variations).. but mostly its color choices, which for us visual people it can make a big difference in appearance on the iPhone screen...
Sure - It's APIs I've not really looked into, so I can't say for sure about the coding aspect either, but I've seen a tech talk from Apple about how they were "now allowing you to change the app icon based on the season without pushing an update", and in that video, the code to change the app icon was based on calendar info, so I kinda assume that what your Appollo app is doing, is a clever workaround hacky thing rather than anything officially supported. You can add several icons to an app's package, but you set which art asset is used as its icon when you upload it to the store, and I don't know how you'd change it in code aside from the aforementioned season change - but maybe user option of icons is also a new addition to iOS 11. When did Appollo start allowing this; Do you know?
Regarding icons, well, you need a regular sized icon, a 2x icon for retina screens, and a 3x version for people using the upscaled version or something like that, so yeah it's not going to take up extreme amounts of storage, but perhaps a few more megs than you'd think.
Agree that colour options for app icons would sometimes be nice though - I've literally colour coded my home screen so each row of icons share the same colour. This means I actually keep a few apps I never use around so I have the colours needed to fill the grid...