upgrade to 20.04 interrupted, third-party ppas reappear after being removed

[Devin Green]

I'm attempting to upgrade to 20.04. I've found lots of people with the same error

An unresolvable problem occurred while calculating the upgrade.

This was likely caused by:
* Unofficial software packages not provided by Ubuntu
Please use the tool 'ppa-purge' from the ppa-purge
package to remove software from a Launchpad PPA and
try the upgrade again.

If none of this applies, then please report this bug using the
command 'ubuntu-bug ubuntu-release-upgrader-core' in a terminal. If
you want to investigate this yourself the log files in
'/var/log/dist-upgrade' will contain details about the upgrade.
Specifically, look at 'main.log' and 'apt.log'.

The advice seems to be to remove third-party ppas. Nowhere on the internet can I find a reliable method for doing this. I'm missing the most crucial piece of information, how do you even know which ppas are third-party? I've found multiple places where people got the same error, and someone said "This ppa is third-party" but literally zero of them explained how they know that, or how I could figure that out. I guess I'm just supposed to memorize a list or something?

I have used the Software Update GUI. I go through, remove each PPA. They reappear when I close it. They sometimes stay gone, but they are restored when the do-release-upgrade command fails.

I've used ppa-purge. For most of the ppas this just doesn't run. I directly write in the URL from sources.list.d, and the computer says it doesn't exist. Even for the ones that don't return that error and claim to have removed the ppa, there is no evidence that anything worked. They are still there when I open the gui. do-release-upgrade still doesn't run.

I found a few places that say I should solve this by removing one thing at a time, try to upgrade, each time and eventually it'll run once I've removed the right thing, but this obviously cannot work if the ppas regenerate.

Am I barking up the wrong tree? Where do I even start to figure out this problem?

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubuntu-release-upgrader-core 1:18.04.38
ProcVersionSignature: Ubuntu 5.4.0-51.56~18.04.1-generic 5.4.65
Uname: Linux 5.4.0-51-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.18
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: ubuntu:GNOME
Date: Tue Oct 13 18:38:00 2020
InstallationDate: Installed on 2020-01-08 (279 days ago)
InstallationMedia: Ubuntu 18.04.3 LTS "Bionic Beaver" - Release amd64 (20190805)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2020-10-14 (0 days ago)

[Devin Green (rabbitbodyguard)]

I have deleted the PPA files directly from sources.lists.d using sudo because purge wouldn't touch them. I get the same error when I try to upgrade.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.