How can I create a common library?

Hi guys,
my question is easy, I think. In my solution, I used to create different projects, for example, one for data logic (layer), business logic, etc.

Now, I have to create a library not only for my projects but also an SDK for third parties. I'm not sure if I create a PCL library, my third parties can use it in shared projects. In this library, I have to call HttpClient and I should use Refit and Polly.

I read on Xamarin Help that Portable Class are obsolete. Really?

Any advice? Thank you in advance.