while there are at least 7 reasons why this is not a good idea... and using a direct usb/network mount is better...
i have to admit... I also have a use case for this feature... ( chroots / lxc / docker volatile images )
added to next release... will require two upgrades to take effect as it's been activated late in the sysupgrade logic (which the first upgrade doesn't have)
ROOTFSEXPAND=1
###################################### rpi4only
################################### discouraged backup and migration of your data is your responsibility
################################### upgrade only not applicable to factory
################################### applies only to unmodified 2 partition defaults
#####-discourages upgrades
#####-internal data migration not possible via sysupgrade -b
#####-requires custom data migration
#####-introduces additional fault points
#####-increases fsck and or boot times
#####-high io may adversely effect core router function
# ROOTFSEXPAND=1
[root@dca632 /usbstick 49°]# df -h
Filesystem Size Used Available Use% Mounted on
/dev/root 29.0G 1.4G 27.5G 5% /
(default-not backported takes effect on first upgrade)
(wont touch a modified disk - hopefully)