Most ISPs filter spoofed IP addresses nowadays[1]. Even if your ISP doesn't prevent it, NAT might. You wouldn't get a whole lot of responses this way, and there'd be a strong bias because of regional differences w.r.t. filtering.

[1]: https://spoofer.caida.org/summary.php

How would you do that, though? The browser has to open a socket to something to do this, after all. And that already is a violation.

You'd just send a UDP packet with a spoofed source address and forget about it. There's no need to open any sort of 2-way connection for data that's only being transmitted in one direction (from browser to metrics server).

You could transmit the telemetry through Tor.

Tor is blocked in some places and viewed as very suspicious in others. If you're already in a place where you're trying not to draw attention to yourself, using Tor might not a good option.

This isnt how tcp/ip works

With udp/ip it would work, if none of the routers on the way to the destination filters spoofed IPs.

Wouldn't spoofing IP addresses risk the data packets being filtered out by ISPs or other upstream network providers? IMO if keeping IP addresses hidden is a concern, it'd be better to use something like TOR.