Old:
 * apt install snmpd
 * grep snmp /etc/passwd
 * apt remove snmpd
 * grep snmp /etc/passwd
 * apt install snmpd
 * grep snmp /etc/passwd


With:
     5.7.3+dfsg-1.8ubuntu3.4 500
        500 http://archive.ubuntu.com/ubuntu bionic-proposed/main amd64 Packages
        100 /var/lib/dpkg/status


# grep snmp /etc/passwd
Debian-snmp:x:112:119::/var/lib/snmp:/bin/false
root@b:~# apt remove snmpd
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following packages were automatically installed and are no longer required:
  libsensors4 libsnmp-base libsnmp30
Use 'apt autoremove' to remove them.
The following packages will be REMOVED:
  snmpd
0 upgraded, 0 newly installed, 1 to remove and 22 not upgraded.
After this operation, 144 kB disk space will be freed.
Do you want to continue? [Y/n] Y
(Reading database ... 37984 files and directories currently installed.)
Removing snmpd (5.7.3+dfsg-1.8ubuntu3.4) ...
Processing triggers for man-db (2.8.3-2ubuntu0.1) ...
root@b:~# grep snmp /etc/passwd
Debian-snmp:x:112:119::/var/lib/snmp:/bin/false

It is stying around as intended.

Purge still clears
root@b:~# apt remove --purge snmpd
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following packages were automatically installed and are no longer required:
  libsensors4 libsnmp-base libsnmp30
Use 'apt autoremove' to remove them.
The following packages will be REMOVED:
  snmpd*
0 upgraded, 0 newly installed, 1 to remove and 22 not upgraded.
After this operation, 0 B of additional disk space will be used.
Do you want to continue? [Y/n] Y
(Reading database ... 37966 files and directories currently installed.)
Purging configuration files for snmpd (5.7.3+dfsg-1.8ubuntu3.4) ...
Processing triggers for systemd (237-3ubuntu10.40) ...
Processing triggers for ureadahead (0.100.0-21) ...
root@b:~# grep snmp /etc/passwd

In comparison install/remove/installing the old version removed the users.

root@b:~# apt install snmpd=5.7.3+dfsg-1.8ubuntu3.3 libsnmp30=5.7.3+dfsg-1.8ubuntu3.3
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Suggested packages:
  snmptrapd
The following NEW packages will be installed:
  snmpd
The following packages will be DOWNGRADED:
  libsnmp30
0 upgraded, 1 newly installed, 1 downgraded, 0 to remove and 22 not upgraded.
Need to get 986 kB of archives.
After this operation, 144 kB of additional disk space will be used.
Do you want to continue? [Y/n] Y
Get:1 http://archive.ubuntu.com/ubuntu bionic-updates/main amd64 libsnmp30 amd64 5.7.3+dfsg-1.8ubuntu3.3 [929 kB]
Get:2 http://archive.ubuntu.com/ubuntu bionic-updates/main amd64 snmpd amd64 5.7.3+dfsg-1.8ubuntu3.3 [57.0 kB]
Fetched 986 kB in 1s (1767 kB/s)
Preconfiguring packages ...
dpkg: warning: downgrading libsnmp30:amd64 from 5.7.3+dfsg-1.8ubuntu3.4 to 5.7.3+dfsg-1.8ubuntu3.3
(Reading database ... 37962 files and directories currently installed.)
Preparing to unpack .../libsnmp30_5.7.3+dfsg-1.8ubuntu3.3_amd64.deb ...
Unpacking libsnmp30:amd64 (5.7.3+dfsg-1.8ubuntu3.3) over (5.7.3+dfsg-1.8ubuntu3.4) ...
Selecting previously unselected package snmpd.
Preparing to unpack .../snmpd_5.7.3+dfsg-1.8ubuntu3.3_amd64.deb ...
Unpacking snmpd (5.7.3+dfsg-1.8ubuntu3.3) ...
Setting up libsnmp30:amd64 (5.7.3+dfsg-1.8ubuntu3.3) ...
Setting up snmpd (5.7.3+dfsg-1.8ubuntu3.3) ...
Created symlink /etc/systemd/system/multi-user.target.wants/snmpd.service → /lib/systemd/system/snmpd.service.
Processing triggers for systemd (237-3ubuntu10.40) ...
Processing triggers for man-db (2.8.3-2ubuntu0.1) ...
Processing triggers for ureadahead (0.100.0-21) ...
Processing triggers for libc-bin (2.27-3ubuntu1) ...
root@b:~# grep snmp /etc/passwd
Debian-snmp:x:112:119::/var/lib/snmp:/bin/false
root@b:~# apt remove snmpd
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following packages will be REMOVED:
  snmpd
0 upgraded, 0 newly installed, 1 to remove and 23 not upgraded.
After this operation, 144 kB disk space will be freed.
Do you want to continue? [Y/n] Y
(Reading database ... 37984 files and directories currently installed.)
Removing snmpd (5.7.3+dfsg-1.8ubuntu3.3) ...
Processing triggers for man-db (2.8.3-2ubuntu0.1) ...
root@b:~# grep snmp /etc/passwd


Thereby this works as intended
block-proposed-bionic is already present, setting verified as well now.