From what I understand, sentry.io is like a tracing and logging service, used by many organizations.

This helps you (=NAS developer) to centralize logs and trace a request through all your application layers (client->server->db and back), so you can identify performance bottlenecks and measure usage patterns.

This is what you can find behind the 'anonymized diagnostics' and 'telemetry' settings you are asked to enable/consent.

For a WebUI it is implemented via javascript, which runs on the client's machine and hooks into the clicks, API calls and page content. It then sends statistics and logs back to, in this case, sentry.io. Your browser just sees javascript, so don't blame them. Privacy Badger might block it.

It is as nefarious as the developer of the application wants to use it. Normally you would use it to centralize logging, find performance issues, and get a basic idea on what features users actually use, so you can debug more easily. But you can also use it to track users. And don't forget, sentry.io is a cloud solution. If you post it on machines outside your control, expect it to be public. Sentry has a self-hosted solution, btw.

My employer uses Sentry for (backend) metrics collection so I had to unblock it to do my job. I wish Sentry would have separate infra for "operating on data collected by Sentry" and "submit every mouse click to Sentry" so I could block their mass surveillance and still do my job, but I suppose that would cut into their profit margins.

My current solution is a massive hack that breaks down every now and then.