One of my hosts runs on Mac OS Catalina, and it constantly runs out of disk space...
I have scheduled tasks running there and every day it uploads files into /Users/labuser/myfolder and removes older files from that folder.
After digging through folders I found that /System/Volumes/Data/Users/labuser/myfolder takes 90% of occupied space on my host.
Is there a way to disable this feature on Catalina and stop it from growing /System/Volumes/Data/... ?
/Users/labuser/myfolder is equivalent to the folder with /System/Volumes/Data/ prepended. macOS 10.15 Catalina added firmlinks (some more description here), but actually from a practical perspective (to the user) these are one and the same.
Thus, your problem has nothing to do with a "feature" on Catalina; rather it has to do with the amount of data you're storing and backing up from /Users/labuser/myfolder.
Whether you use ncdu or another disk usage manager that will solve you problem of finding out why you're consuming all of your disk space.
One other relevant point is that because these are "symlinked" (called firmlinks by Apple), some disk inventory apps don't know how to handle this and end up in a recursion scenario when trying to understand total disk usage. I've seen this behavior with ncdu also. That being said, if you run the disk inventory on a subfolder of /System/Volumes/Data/, e.g.:
cd /Users
ncdu
It should avoid these issues.