Hello,
I'm running Kolab16 on Debian 11, and php-net-sieve is also available from Debian:
# apt policy php-net-sieve php-net-sieve: Installiert: 1.4.4-0~kolab2 Installationskandidat: 1.4.4-0~kolab2 Versionstabelle: 1.4.4-2 500 500 http://mirror.netcologne.de/debian bullseye/main amd64 Packages *** 1.4.4-0~kolab2 510 510 https://mirror.apheleia-it.ch/repos/Kolab:/16/Debian_11.0 ./ Packages 100 /var/lib/dpkg/status
I looked at the kolab source and I don't see kolab specific changes in the diff.
My guess is that we should be able to drop the kolab package.