Channels in this range are typically paying to receive access on Comcast. When they stop paying, Comcast stops carrying them. This is likely what happened with RT in this case, which is usually being fed from a local market station of some sort, usually a low-power affiliate.
I am really upset that Comcast dropped RT! And not because I support the pro or anti Russia narrative. It just offers a fresh perspective that other media outlets lack. Bring it back or I am much more inclined to cut the cord!
As mentioned, RT was typically on a LEASED channel. That means RT was paying Comcast to carry it. Unless you have proof that Comcast refused to renew the lease, then it's more like RT didn't want to pay anymore.