I think the issue with Openwrt 21.02.0-rc2 may have to do with libusb. When I type lsusb -t to show the tree of the USB connections, I get the following which shows the iPhone connected:
/: Bus 04.Port 1: Dev 1, Class=root_hub, Driver=xhci-hcd/0p, 5000M
/: Bus 03.Port 1: Dev 1, Class=root_hub, Driver=xhci-hcd/1p, 480M
/: Bus 02.Port 1: Dev 1, Class=root_hub, Driver=xhci-hcd/1p, 5000M
/: Bus 01.Port 1: Dev 1, Class=root_hub, Driver=xhci-hcd/1p, 480M
|__ Port 1: Dev 3, If 0, Class=, Driver=, 480M
The last line (Port 1, Dev 3, IF 0) is the iPhone. However, the Class= and Driver= are blank, which makes me think this version of libusb isn't recognizing the device to pass to usbmuxd. Previously on 19.07.7 that showed Class=Vendor Specific Class or something similar. On 21.02.0-rc2 it is blank.
Here is the same output running on 19.07.7:
/: Bus 04.Port 1: Dev 1, Class=root_hub, Driver=xhci-hcd/0p, 5000M
/: Bus 03.Port 1: Dev 1, Class=root_hub, Driver=xhci-hcd/1p, 480M
/: Bus 02.Port 1: Dev 1, Class=root_hub, Driver=xhci-hcd/1p, 5000M
/: Bus 01.Port 1: Dev 1, Class=root_hub, Driver=xhci-hcd/1p, 480M
|__ Port 1: Dev 2, If 0, Class=Imaging, Driver=, 480M
|__ Port 1: Dev 2, If 1, Class=Vendor Specific Class, Driver=usbfs, 480M
|__ Port 1: Dev 2, If 2, Class=Vendor Specific Class, Driver=ipheth, 480M
On 19.07.7, it shows the Vendor Specific Class and ipheth Driver.
Back on 21.02.0, when I try to get more details on the iPhone using lsusb -D /dev/bus/usb/001/003, I get:
Cannot open /dev/bus/usb/001/003
That same command returns a bunch of information about the iPhone on Openwrt 19.07.7.