Open In Webmail, being the tiny utility app that it is, collects no data on your usage and transmits no data on your usage. In fact, it opts into additional macOS sandboxing such that it is unable to make network requests.
It’s a very simple app — macOS launches Open In Webmail whenever you open a “mailto:” link; Open In Webmail then opens an “https://” URL directly to your web mail service of choice.
This is never expected to change for the life of this application.
Effective January 31st, 2019.