If Apple is going to support AV1, it starts with decoding. Apple isn't going to give their phone the ability to record videos in a format that the desktop OS and the desktop computers don't even support.

Once AV1 decoding is present across the product line, I imagine the phones might get support for AV1 encoding, probably two generations down the line, possibly next generation. Still a fairly large if though, given that Apple is decently invested in h265.

They have too many choices already. Even if they do add an encoder at a later iteration, it will likely only be for video conferencing. Too small to care about really, given that those transistors can used for something else.