The oldest supported release of Lubuntu is Lubuntu 20.04 LTS.
I’ll provide some relevant links
- https://lubuntu.me/bionic-eol/
- Lubuntu 18.04 LTS End of Life (30-April-2021)
- Lubuntu 18.04 LTS is nearing it's EOL (end-of-life)]
- https://lubuntu.me/bionic-5-released/
As such I’ve moved this post to off-topic, as the release is unsupported by Lubuntu.
Your system in my view is now a Ubuntu 18.04 LTS (it has 5 years of support, flavors do not) with the LXDE desktop (as was used by Lubuntu 18.04 LTS).
I’d suggest using ubuntu-support-status to confirm the supported status on your actual install to evaluate the security status of your own system with your actual usage of it. See the aforementioned discourse posts for examples.
Many support sites still support 18.04; but you risk getting reminders that Lubuntu 18.04 LTS is EOL, thus using Ubuntu 18.04 LTS (with LXDE) maybe easier.