I think what you are getting at is that websockets aren't as simple as http traffic through a proxy, but you absolutely can use proxies and ws connections just fine and for a variety of reasons.

For all the other comments, parent is probably talking about forward proxies and to their point many forward/enterprise proxies have configurations which cause websockets to break and it is a pain to debug this if you have many enterprise customers.

Echoing this. At $DAYJOB some 5-10% of customers will fail to initiate a websocket connection, even over wss:// despite plain HTTPS requests working fine. This is a client-side issue with whatever outdated HTTP CONNECT implementation the enterprise has.

Works completely fine in Haproxy

This isn't based on any facts

I've definitely used websockets through nginx

They even go through Cloudflare.

or Fly.io

Says who?

I use them though nginx/cloudflare. they work fine.