systemd rejects cross-uid-namespace connections

To reproduce:

    $ unshare -r systemd-run --user echo it works
    Failed to start transient service unit: Access denied

There is prior-art in the RHEL-8 bug report and the fix:

https://bugzilla.redhat.com/show_bug.cgi?id=1838081#c13

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: systemd 237-3ubuntu10.42
ProcVersionSignature: Ubuntu 4.15.0-117.118-generic 4.15.18
Uname: Linux 4.15.0-117-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.17
Architecture: amd64
CurrentDesktop: Unity
Date: Wed Sep 30 10:12:45 2020
InstallationDate: Installed on 2020-09-14 (15 days ago)
InstallationMedia: Ubuntu 18.04.5 LTS "Bionic Beaver" - Release amd64 (20200806.1)
MachineType: Hewlett-Packard p2-1320a
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-4.15.0-117-generic root=/dev/mapper/root--vg-lvroot ro quiet splash vt.handoff=1
SourcePackage: systemd
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 08/29/2012
dmi.bios.vendor: AMI
dmi.bios.version: 8.08
dmi.board.asset.tag: 3CR2450BRJ
dmi.board.name: 2AE2
dmi.board.vendor: PEGATRON CORPORATION
dmi.board.version: 1.02
dmi.chassis.asset.tag: 3CR2450BRJ
dmi.chassis.type: 3
dmi.chassis.vendor: Hewlett-Packard
dmi.modalias: dmi:bvnAMI:bvr8.08:bd08/29/2012:svnHewlett-Packard:pnp2-1320a:pvr1.00:rvnPEGATRONCORPORATION:rn2AE2:rvr1.02:cvnHewlett-Packard:ct3:cvr:
dmi.product.family: 103C_53316J G=D
dmi.product.name: p2-1320a
dmi.product.version: 1.00
dmi.sys.vendor: Hewlett-Packard