Is there a way we can detect is someone uninstalled or stopped the osquery agent on a host?

One easy way I can think of is: In the

Hosts

tab, select

Edit columns

, select only

Last fetched

, then click on

Export hosts

, and finally filter the exported CSV by inspecting. the "Last fetched" timestamp. "Last fetched" is the last time Fleet was able to ingest data from the host.