This is still present in Bionic.

$ LC_ALL=C apt-cache policy zsync
zsync:
Installed: 0.6.2-3ubuntu1
Candidate: 0.6.2-3ubuntu1
Version table:
*** 0.6.2-3ubuntu1 500
500 http://fi.archive.ubuntu.com/ubuntu bionic/universe amd64 Packages
100 /var/lib/dpkg/status
$ zsync http://cdimage.ubuntu.com/ubuntu-server/daily/current/bionic-server-amd64.iso.zsync
#################### 100.0% 0.0 kBps DONE

No relevent local data found – I will be downloading the whole file. If that’s not what you want, CTRL-C out. You should specify the local file is the old version of the file to download with -i (you might have to decompress it with gzip -d first). Or perhaps you just have no data that helps download the file
downloading from http://cdimage.ubuntu.com/ubuntu-server/daily/current/bionic-server-amd64.iso:
#################### 100.0% 20540.8 kBps DONE

verifying download…checksum matches OK