Is it safety to erase the /etc/apt/sources.list.d folder files

The file /etc/apt/sources/sources.list, and the .list files in /etc/apt/sources.list.d, work together to provide information to apt about software repositories and PPA locations. These files are used when Software Updater scans for newer applications, newer kernels, and newer libraries for your system.

If you're knowledgeable about how all of this works, then yes, you can manually edit/delete .list files in /etc/apt/sources.list.d. But because you're asking the question, I suspect that this is new territory for you. So don't do this manually.

I'd recommend using the proper GUI application, Software & Updates, "Other Software" tab, to manage the .list files that you see in /etc/apt/sources.list.d. There, you can examine, enable/disable, edit, or remove .list files.

For each entry, the goal is to have current Ubuntu generations in the .list file, like focal, IF the repository or PPA supports such a new version. Some still only support eoan, or older. So you have to know how to determine what each repository or PPA site supports. You generally don't want repositories or PPAs that support older versions than what you're currently running on your computer, as the software may not be compatible with your current system.

Any changes that you make via Software & Updates will trigger a reload of the software databases when you click the Close button. Any errors that appear indicate that you've made a mistake in one of your edits.