This is really reaching. Channelization as being referred to upthread in reference to SSH or RDP is a protocol level abstraction, rs485 bussing is at the electrical level for multiple physical devices and also doesn’t specify any addressing scheme, that would be specified as a higher level protocol (that could be done just as well over rs-232). Device addressing and channelization are not even logically the same thing.

Ethernet as it is typically deployed is not a bus either but it is used in channelized communication (packet switched no less) just fine.

Edit: responding to your edit. The point still stands this is orthogonal to the physical or electrical layer. The vast vast majority of the internet including Ethernet and fiber links are point to point and clearly they carry a tremendous number of separate streams, and a small amount of broadcast. You can run layer 2 over an RS-232C serial port, whether its Ethernet, PPP, SLIP or ATM. In Linux PPP is in fact implemented as a tty line discipline.

As for RS-485, there are widely used standards such as DMX512. It is not a strongly supported idea to put this stuff in the kernel, when performance in user space is sufficient.

Finally, RS-232 can do the same thing (multidrop) with electrical repeaters. See MAX3322 eg.