There are apps in /data and others in /system
Deleting apps from /system can be done manually as well, but needs a remount as writable of course. This will free up space on /system that can be used to move apps from /data to /system which in turn frees up space on /data which in turn lets you install more apps to /data
But take care! The filesystem easlily gets corrupted and there is no regular checks on a stock T1, read here for more:
https://www.mobileread.com/forums/sho...d.php?t=166723