Well, I took a look at @bryan's answer and the succinct answer he proposed. My interpretation of it is thus. First, the behavior is caused by the use of the -X flag. From the text,
The warning can be silenced by using ssh -Y, since that is what ssh -X
is doing now anyway.
For an explanation of why that may work, read the link. Specifically, the reasoning behind these warnings is
Trusted X11 forwarding means that you trust the server that you wish
to ssh into is not using any keyloggers, screenshot utilities, packet
sniffers, or anything else to hijack your connection, in which case
X11 will allow it to do whatever a local client would be able to do.
so the -Y is providing this level of trust to avoid warnings. However, that may not be sufficient. I, for example, was trying to silence this message on a Rocks cluster where changes had to be done at the configuration level. You can also silence it via the ssh_config file. According to this link you can add
into /etc/ssh/ssh_config. However there are ramifications to doing so. I am running on a secure, private network so the concerns are limited. I will also probably do this on my laptop as well, but I never said I fully understood the concerns myself! If you can express why this may still be risky (I only ssh into computers I know fairly well) then please help clarify in comments.