It isn't unknown for software to have problems in the OTHER direction with the Proxy header. Guzzle had this, where you could set a Proxy request header and all the curl requests would try to use the address you sent for it's calls.

Any header the user can set is always suspect.

We 'use' it when the ALB sets it for real-client-ip in nginx (while understanding that if something looks weird we should look at the full header). However a better solution is if your edge service sets (and protects, that's important) a header with the client IP to use that.

Cloudflare does this. Fastly does this as well, and I know Fastly protects it because I tested this specifically.

Cloudflare actually can set it in two different ways, however one of the ways is an upsell (only because it names it Real-Client-Ip which is apparently something set by other products, including Fastly). I was very amused when I got the Cloudflare rep to admit that.

Depends, some load balancers may think they are behind another load balancer, and forward the header. This is default behavior iirc.