Signal actually does optionally offer previews for a handful of services, and they really jump through some hoops to make that safer:

https://support.signal.org/hc/en-us/articles/360022474332-Li...

The service being previewed doesn't know who you are because Signal acts as a proxy, Signal doesn't know what you previewed on that service because their client deliberately sends overlapping Range requests so that the preview size is rounded.