2.3.1-2.1ubuntu4 is the version of libseccomp2 which is available, i.e.
$ sudo apt install libseccomp2=2.3.1-2.1ubuntu4
succeeds. That allowed me to uncomment, i.e. enable, the SystemCallFilter in /lib/systemd/system/systemd-resolved.service again and reboot successfully.
2.3.1-2.1ubuntu4 is the version of libseccomp2 which is available, i.e. 2.3.1-2. 1ubuntu4 system/ systemd- resolved. service again and reboot successfully.
$ sudo apt install libseccomp2=
succeeds. That allowed me to uncomment, i.e. enable, the SystemCallFilter in /lib/systemd/