Distribution?
Manually tried to install the package? (apt yum dnf...)
It's definetly not a bug, it's a feature request (plugin).
It shouldn't be installed automatically, but mentioned as recommended.

https://plugins.roundcube.net/#/

My Linux distribution is Debian 10, the roundcubemail-plugin-contextmenu package is no longer available for installation using apt, after you upgraded roundcube from 1:1.4.13-0~kolab3 to 1:1.5.3-1~kolab3 (before it was available and it was actually automatically installed during the kolab installation).

But I understand your point, I considered it a bug as it was available before 'out of the box' as I mentioned. I guess with the roundcube upgrade now I need to install it by myself, I will try with the instructions you provided (PHP Composer).

Feel free to close the issue and thanks for the help!

you could try direct with apt install
it is in repository!
https://obs.kolabsys.com/repositories/Kolab:/16/Debian_10.0/all/roundcubemail-plugin-contextmenu_3.0-0~kolab13_all.deb

That is not an option I am afraid, as apt will then propose to downgrade roundcubemail and remove chwala, irony, etc.
This is actually what I meant with the ticket... there is a new package called 'roundcubemail-plugins-kolab' that seems to supersede all individual previous plugins.

apt install roundcubemail-plugin-contextmenu

The following additional packages will be installed:

php-net-sieve roundcubemail-core roundcubemail-plugin-acl roundcubemail-plugin-archive
roundcubemail-plugin-filesystem-attachments roundcubemail-plugin-jqueryui roundcubemail-plugin-managesieve
roundcubemail-plugin-newmail-notifier roundcubemail-plugin-password roundcubemail-plugin-redundant-attachments
roundcubemail-plugin-zipdownload

The following packages will be REMOVED:

chwala irony kolab kolab-freebusy kolab-syncroton kolab-webclient roundcubemail roundcubemail-plugins-kolab
roundcubemail-skin-chameleon

The following NEW packages will be installed:

php-net-sieve roundcubemail-core roundcubemail-plugin-acl roundcubemail-plugin-archive
roundcubemail-plugin-contextmenu roundcubemail-plugin-filesystem-attachments roundcubemail-plugin-jqueryui
roundcubemail-plugin-managesieve roundcubemail-plugin-newmail-notifier roundcubemail-plugin-password
roundcubemail-plugin-redundant-attachments roundcubemail-plugin-zipdownload

0 upgraded, 12 newly installed, 9 to remove and 0 not upgraded.

I will try with PHP Composer to install it manually.