As 18.04 is relatively new, not all repositories (including PPAs) are releasing their packages for Bionic. Still, there are constantly questions regarding 404 error while trying to install package(s) from PPAs.
In short, I want to discuss: What to do with questions regarding 404 error on repositories while the original poster wanted to install that package? This question have only answers to remove PPAs.
This can be a future-proofing, so new users can know what to do when they're still want to install a certain package, with knowledge of risk (if there's any) if they want to install the older version for older Ubuntu release.