At the moment, our plugins run on agents and they can only execute calls locally to where they are running. We are working on a new feature which will allow us to run plugins externally which would then satisfy your use case.
You mentioned some years ago about that Dynatrace is working on a new feature but I'm wondering if any Plugins was created already, we also want to perform availability check on devices where it is not possible to
As I mentioned in my earlier comment, at the moment no external pings can be done with plugins. This is planned for early next year and will allow you to execute plugins towards targets that cannot be instrumented with a OneAgent. In a first iteration this could allow us to have support for known devices such as an F5. Later-on that can be extended.
You can also perform the pings outside Dynatrace environment and just send the availability/performance data into Dynatrace using API. More details here:
However the execution itself is up to you.
There is in Dynatrace Hub a synthetic monitor ping that you can use.
Just some additional two cents on this: the extensibility of Dynatrace is very good, and in our case we have even built an extension that pings with 1 second resolution, enabling the possibility of having strong network SLA measuring, in both latency & availability. I'm in the process of trying to solve a dependency issue, to make it available.