Like within the header? It will show the custom domain in the from field of the email. Im not sure what the big deal is, but I’m curious.
The "via forwardmail" part is subtle enough that it's not a big deal for me and if you start looking out for it, you'll see it a lot.
EDIT: I was trying out Cloudflare email routing yesterday so I checked one of the test emails I sent. There's no "via Cloudflare" visible, and actually it's sent from smtp.gmail.com so that's probably reputable enough to get high deliverability. All I see in the from field is "abc@mysite.com".