After encountering the reported bug in 3.16 + Qt 5.9.5 (https://github.com/qgis/QGIS/issues/39693), I tried to remove and reinstall and got the broken dependencies below.
user@dell: sudo apt install qgis qgis-plugin-grass
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:
The following packages have unmet dependencies:
qgis : Depends: libgdal20 (>= 1.8.0) but it is not going to be installed
Depends: libqgis-3d3.16.0 but it is not going to be installed
Depends: libqgis-analysis3.16.0 but it is not going to be installed
Depends: libqgis-app3.16.0 but it is not going to be installed
Depends: libqgis-core3.16.0 but it is not going to be installed
Depends: libqgis-gui3.16.0 but it is not going to be installed
Depends: python3-qgis (= 1:3.16.0+28bionic) but it is not going to be installed
Depends: qgis-providers (= 1:3.16.0+28bionic) but it is not going to be installed
qgis-plugin-grass : Depends: qgis-provider-grass (= 1:3.16.0+28bionic) but it is not going to be installed
Depends: grass740
Depends: libgdal20 (>= 1.8.0) but it is not going to be installed
Depends: libqgis-app3.16.0 but it is not going to be installed
Depends: libqgis-core3.16.0 but it is not going to be installed
Depends: libqgis-gui3.16.0 but it is not going to be installed
Depends: libqgisgrass7-3.16.0 but it is not going to be installed
E: Unable to correct problems, you have held broken packages.
System is Ubuntu 18.04. Sources are:
# deb cdrom:[Ubuntu 18.04 _Bionic_ - Build amd64 LIVE Binary 20180608-09:38]/ bionic main
See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
newer versions of the distribution.
deb http://il.archive.ubuntu.com/ubuntu/ bionic main restricted
deb-src http://il.archive.ubuntu.com/ubuntu/ bionic main restricted
Major bug fix updates produced after the final release of the
distribution.
deb http://il.archive.ubuntu.com/ubuntu/ bionic-updates main restricted
deb-src http://il.archive.ubuntu.com/ubuntu/ bionic-updates main restricted
N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
team. Also, please note that software in universe WILL NOT receive any
review or updates from the Ubuntu security team.
deb http://il.archive.ubuntu.com/ubuntu/ bionic universe
deb-src http://il.archive.ubuntu.com/ubuntu/ bionic universe
deb http://il.archive.ubuntu.com/ubuntu/ bionic-updates universe
deb-src http://il.archive.ubuntu.com/ubuntu/ bionic-updates universe
N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
team, and may not be under a free licence. Please satisfy yourself as to
your rights to use the software. Also, please note that software in
multiverse WILL NOT receive any review or updates from the Ubuntu
security team.
deb http://il.archive.ubuntu.com/ubuntu/ bionic multiverse
deb-src http://il.archive.ubuntu.com/ubuntu/ bionic multiverse
deb http://il.archive.ubuntu.com/ubuntu/ bionic-updates multiverse
deb-src http://il.archive.ubuntu.com/ubuntu/ bionic-updates multiverse
N.B. software from this repository may not have been tested as
extensively as that contained in the main release, although it includes
newer versions of some applications which may provide useful features.
Also, please note that software in backports WILL NOT receive any review
or updates from the Ubuntu security team.
deb http://il.archive.ubuntu.com/ubuntu/ bionic-backports main restricted universe multiverse
deb-src http://il.archive.ubuntu.com/ubuntu/ bionic-backports main restricted universe multiverse
Uncomment the following two lines to add software from Canonical's
'partner' repository.
This software is not part of Ubuntu, but is offered by Canonical and the
respective vendors as a service to Ubuntu users.
deb http://archive.canonical.com/ubuntu bionic partner
deb-src http://archive.canonical.com/ubuntu bionic partner
deb http://security.ubuntu.com/ubuntu bionic-security main restricted
deb-src http://security.ubuntu.com/ubuntu bionic-security main restricted
deb http://security.ubuntu.com/ubuntu bionic-security universe
deb-src http://security.ubuntu.com/ubuntu bionic-security universe
deb http://security.ubuntu.com/ubuntu bionic-security multiverse
deb-src http://security.ubuntu.com/ubuntu bionic-security multiverse
deb https://qgis.org/ubuntu bionic main
deb-src https://qgis.org/ubuntu bionic main
deb [arch=amd64] https://download.docker.com/linux/ubuntu bionic stable
deb-src [arch=amd64] https://download.docker.com/linux/ubuntu bionic stable
deb https://qgis.org/ubuntu-ltr bionic main
deb-src https://qgis.org/ubuntu-ltr bionic main```
How can I revert to 3.10 ? I tried to replace
deb https://qgis.org/ubuntugis bionic maindeb-src https://qgis.org/ubuntugis bionic mainwith
deb https://qgis.org/ubuntugis-ltr bionic maindeb-src https://qgis.org/ubuntugis-ltr bionic mainbut hat did not help. Version 3.16 remains. Any pointers? Tnx!
– gastgis Nov 19 '20 at 08:10