I believe the explanation your question was closed is, that you asked about a [specific ppa](https://askubuntu.com/questions/1398568/installing-python-who-is-deadsnakes-and-why-should-i-trust-them), but this was already answered in a [generic way](https://askubuntu.com/questions/35629/are-ppas-safe-to-add-to-my-system-and-what-are-some-red-flags-to-watch-out-for).

The reasoning here is, that from the generic information about green and red flags for ppa's you should be able to extrapolate this information, and apply it to your specific case.

For instance, the ppa `deadsnakes` has a [Github page](https://github.com/deadsnakes), which is updated daily.

In addition, the [ppa itself](https://launchpad.net/~deadsnakes/+archive/ubuntu/ppa) is very active, there is detailed information, support for both 18.04 and 20.04, and the main author ([Felix Krull](https://launchpad.net/~fkrull)) has been a member since 2009.

All this points to a legitimate and good supported ppa.

Just imagine if the same question should be asked for each and every ppa in existence - this would give hundreds, maybe even thousands of questions with basically the same answer: Engage in your own thorough due diligence.