@flohack I was able to boot into that image and list the data partition:
agates@risa ~/D/ubports> fastboot flash boot boot_2021-09-03.img
Sending 'boot_a' (32768 KB) OKAY [ 0.777s]
Writing 'boot_a' OKAY [ 0.155s]
Finished. Total time: 0.937s
agates@risa ~/D/ubports> adb devices
* daemon not running; starting now at tcp:5037
* daemon started successfully
List of devices attached
FA79F1A09215 recovery
agates@risa ~/D/ubports> adb shell
root@walleye:/ # mount /data
root@walleye:/ # ls -lh /data
total 2.0K
drwx------ 4 root root 4.0K 1972-09-14 01:27 android-data
root@walleye:/ # mount
rootfs on / type rootfs (rw)
tmpfs on /dev type tmpfs (rw,nosuid,relatime,size=1853348k,nr_inodes=463337,mode=755)
devpts on /dev/pts type devpts (rw,relatime,mode=600,ptmxmode=000)
proc on /proc type proc (rw,relatime,gid=3009,hidepid=2)
sysfs on /sys type sysfs (rw,relatime)
tmpfs on /mnt type tmpfs (rw,nosuid,nodev,noexec,relatime,size=1853348k,nr_inodes=463337,mode=755,gid=1000)
none on /acct type cgroup (rw,relatime,cpuacct)
tmpfs on /tmp type tmpfs (rw,relatime,size=1853348k,nr_inodes=463337)
tmpfs on /storage type tmpfs (rw,relatime,size=1853348k,nr_inodes=463337,mode=050,gid=1028)
pstore on /sys/fs/pstore type pstore (rw,relatime)
none on /config type configfs (rw,relatime)
adb on /dev/usb-ffs/adb type functionfs (rw,relatime)
/dev/block/sda45 on /cache type ext4 (rw,relatime,errors=remount-ro,stripe=4096,data=ordered)
/dev/block/sda45 on /data type ext4 (rw,relatime,errors=remount-ro,stripe=4096,data=ordered)