USB serial devices not showing in /dev/serial/by-id/ after update from 21.04 to 21.10

TL;DR: Please check if linux-modules-extra-raspi package is installed. Looks like some drivers were moved into a new package in 21.10 to reduce the size of the initramfs. Installing it manually helped in my case:

sudo apt install linux-modules-extra-raspi
sudo reboot

I am also affected by the exact same issue. Earlier today, I was working and setting up an USB IR reader for my power meter with Ubuntu 21.04 on a Raspberry Pi 4. After do-release-upgrade, the /dev/ttyUSB0 did not show up. The USB device in lsusb is there, but somehow the usbserial device creation does not kick in.

Bus 003 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 002 Device 002: ID 174c:1153 ASMedia Technology Inc. ASM1153 SATA 3Gb/s bridge
Bus 002 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
Bus 001 Device 003: ID 0403:6015 Future Technology Devices International, Ltd Bridge(I2C/SPI/UART/FIFO)
Bus 001 Device 002: ID 2109:3431 VIA Labs, Inc. Hub
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub

Device in question is 0403:6015.

There seem to be a related bug filed here already on launchpad.