If I drag the "Install" button from the Marketplace website to my Eclipse IDE (2019-09) application window I see an error message saying:
"Unable to install selected solutions
The following solutions are not compatible with this version of Eclipse:
StartExplorer"
At the same time installation via the P2 update site works fine.
I hope this is only an issue with the metadata, since only versions up to "2018-09 (4.9)" are marked as supported (https://marketplace.eclipse.org/content/startexplorer -> additional details).
=> Please enable all Eclipse versions up to 2019-09 (4.13) as supported.