I don't think flash was in the original package. On my 5.2 machine, it looks like flashplugin-installer first got installed as part of a routine update I ran first thing on 2020-12-09.
Code:
Start-Date: 2020-12-09 06:59:28
Commandline: apt-get -o Dpkg::Options::=--force-confdef -o Dpkg::Options::=--force-confold dist-upgrade -y
Requested-By: stephen (1000)
Upgrade: openssl:amd64 (1.1.1f-1ubuntu2, 1.1.1f-1ubuntu2.1), gir1.2-gdkpixbuf-2.0:amd64 (2.40.0+dfsg-3, 2.40.0+dfsg-3ubuntu0.1), libgdk-pixbuf2.0-0:amd64 (2.40.0+dfsg-3, 2.40.0+dfsg-3ubuntu0.1), libgdk-pixbuf2.0-0:i386 (2.40.0+dfsg-3, 2.40.0+dfsg-3ubuntu0.1), libparted2:amd64 (3.3-4, 3.3-4ubuntu0.20.04.1), python3-aptdaemon.gtk3widgets:amd64 (1.1.1+bzr982-0ubuntu32.2, 1.1.1+bzr982-0ubuntu32.3), flashplugin-installer:amd64 (32.0.0.453ubuntu0.20.04.1, 32.0.0.465ubuntu0.20.04.1), libparted-fs-resize0:amd64 (3.3-4, 3.3-4ubuntu0.20.04.1), parted:amd64 (3.3-4, 3.3-4ubuntu0.20.04.1), libgdk-pixbuf2.0-bin:amd64 (2.40.0+dfsg-3, 2.40.0+dfsg-3ubuntu0.1), libgdk-pixbuf2.0-common:amd64 (2.40.0+dfsg-3, 2.40.0+dfsg-3ubuntu0.1), aptdaemon-data:amd64 (1.1.1+bzr982-0ubuntu32.2, 1.1.1+bzr982-0ubuntu32.3), python3-aptdaemon:amd64 (1.1.1+bzr982-0ubuntu32.2, 1.1.1+bzr982-0ubuntu32.3), aptdaemon:amd64 (1.1.1+bzr982-0ubuntu32.2, 1.1.1+bzr982-0ubuntu32.3), linux-firmware:amd64 (1.187.4, 1.187.6), libssl1.1:amd64 (1.1.1f-1ubuntu2, 1.1.1f-1ubuntu2.1), libssl1.1:i386 (1.1.1f-1ubuntu2, 1.1.1f-1ubuntu2.1)
End-Date: 2020-12-09 07:03:20
I didn't notice it at the time and purged it on December 18 having read Moltke's post.