Confirmed this has no reverse-dependancies, and is not seeded.  We have a bit of a mess in bionic-proposed because of the iproute transition, therefore it needs removing there too:

Removing packages from bionic-proposed:
        percona-xtradb-cluster-5.6 5.6.37-26.21-0ubuntu3 in bionic
                percona-xtradb-cluster-5.6-dbg 5.6.37-26.21-0ubuntu3 in bionic arm64
                percona-xtradb-cluster-5.6-dbg 5.6.37-26.21-0ubuntu3 in bionic armhf
                percona-xtradb-cluster-5.6-dbg 5.6.37-26.21-0ubuntu3 in bionic i386
                percona-xtradb-cluster-5.6-dbg 5.6.37-26.21-0ubuntu3 in bionic ppc64el
                percona-xtradb-cluster-5.6-dbg 5.6.37-26.21-0ubuntu3 in bionic s390x
                percona-xtradb-cluster-server-5.6 5.6.37-26.21-0ubuntu3 in bionic arm64
                percona-xtradb-cluster-server-5.6 5.6.37-26.21-0ubuntu3 in bionic armhf
                percona-xtradb-cluster-server-5.6 5.6.37-26.21-0ubuntu3 in bionic i386
                percona-xtradb-cluster-server-5.6 5.6.37-26.21-0ubuntu3 in bionic ppc64el
                percona-xtradb-cluster-server-5.6 5.6.37-26.21-0ubuntu3 in bionic s390x
                percona-xtradb-cluster-source-5.6 5.6.37-26.21-0ubuntu3 in bionic arm64
                percona-xtradb-cluster-source-5.6 5.6.37-26.21-0ubuntu3 in bionic armhf
                percona-xtradb-cluster-source-5.6 5.6.37-26.21-0ubuntu3 in bionic i386
                percona-xtradb-cluster-source-5.6 5.6.37-26.21-0ubuntu3 in bionic ppc64el
                percona-xtradb-cluster-source-5.6 5.6.37-26.21-0ubuntu3 in bionic s390x
Comment: Replaced by percona-xtradb-cluster-5.7; LP: #1752373
1 package successfully removed.

Removing packages from bionic:
        percona-xtradb-cluster-5.6 5.6.37-26.21-0ubuntu2 in bionic
                percona-xtradb-cluster-5.6-dbg 5.6.37-26.21-0ubuntu2 in bionic amd64
                percona-xtradb-cluster-5.6-dbg 5.6.37-26.21-0ubuntu2 in bionic arm64
                percona-xtradb-cluster-5.6-dbg 5.6.37-26.21-0ubuntu2 in bionic armhf
                percona-xtradb-cluster-5.6-dbg 5.6.37-26.21-0ubuntu2 in bionic i386
                percona-xtradb-cluster-5.6-dbg 5.6.37-26.21-0ubuntu2 in bionic ppc64el
                percona-xtradb-cluster-5.6-dbg 5.6.37-26.21-0ubuntu2 in bionic s390x
                percona-xtradb-cluster-server 5.6.37-26.21-0ubuntu2 in bionic amd64
                percona-xtradb-cluster-server 5.6.37-26.21-0ubuntu2 in bionic arm64
                percona-xtradb-cluster-server 5.6.37-26.21-0ubuntu2 in bionic armhf
                percona-xtradb-cluster-server 5.6.37-26.21-0ubuntu2 in bionic i386
                percona-xtradb-cluster-server 5.6.37-26.21-0ubuntu2 in bionic ppc64el
                percona-xtradb-cluster-server 5.6.37-26.21-0ubuntu2 in bionic s390x
                percona-xtradb-cluster-server-5.6 5.6.37-26.21-0ubuntu2 in bionic amd64
                percona-xtradb-cluster-server-5.6 5.6.37-26.21-0ubuntu2 in bionic arm64
                percona-xtradb-cluster-server-5.6 5.6.37-26.21-0ubuntu2 in bionic armhf
                percona-xtradb-cluster-server-5.6 5.6.37-26.21-0ubuntu2 in bionic i386
                percona-xtradb-cluster-server-5.6 5.6.37-26.21-0ubuntu2 in bionic ppc64el
                percona-xtradb-cluster-server-5.6 5.6.37-26.21-0ubuntu2 in bionic s390x
                percona-xtradb-cluster-source-5.6 5.6.37-26.21-0ubuntu2 in bionic amd64
                percona-xtradb-cluster-source-5.6 5.6.37-26.21-0ubuntu2 in bionic arm64
                percona-xtradb-cluster-source-5.6 5.6.37-26.21-0ubuntu2 in bionic armhf
                percona-xtradb-cluster-source-5.6 5.6.37-26.21-0ubuntu2 in bionic i386
                percona-xtradb-cluster-source-5.6 5.6.37-26.21-0ubuntu2 in bionic ppc64el
                percona-xtradb-cluster-source-5.6 5.6.37-26.21-0ubuntu2 in bionic s390x
Comment: Replaced by percona-xtradb-cluster-5.7; LP: #1752373
1 package successfully removed.