> 𝕏 is a pre-existing symbol used by
"used by" is pretty debatable. Its a fairly obscure character. I don't think being in unicode by itself is sufficient enough to count.
There is also https://www.x.org/ which seems quite slow. I wonder if it's receiving more traffic than usually..
𝕏 is a pre-existing symbol used by, and part of a standard character set for, software. You could probably get a trademark on it for a company selling apples, but I highly doubt you could for a software company.