The widespread module "MCP2515_CAN has only 2 connections, CANH and CANL. The CAN-GND required in all serious descriptions is missing. Isn't that necessary in hobby circles with its 20 cm long / short CAN buses?"
If you are asking "is a ground necessary in a system that uses differential drivers" here's a quote from a Texas Instruments application note https://www.ti.com/lit/pdf/snla049
"Although the potential difference between the data-pair conductors determines the signal without officially involving ground, the bus needs a ground wire to provide a return path for induced common-mode noise and currents, such as the receivers' input current. A typical mistake is to connect two nodes with only two wires. If you do this, the system may radiate high levels of EMI, because the common-mode return current finds its way back to the source, regardless of where the loop takes it. An intentional ground provides a low-impedance path in a known location, thus reducing emissions."
There is also the issue of receiver common mode voltage limits so you need to make sure that your grounding scheme design doesn't cause problems.
I don't think I have ever seen a hobby grade anything that has a sticker that shows it has passed any kind of EMI testing.