Is it possible to configure the Ignition OPC UA Server to share only a specific device folder and, within that device, only certain tags? For example, I would like to share only the '[GenSim]' device and, within this device, only the 'Ramp2' tag.
To share with whom or with what?
No, there's no per-device security. When exposing tags, there are some per-tag-provider options, IIUC.
This one of the reasons I offer a Modbus Server driver. You can expose just Modbus TCP instead of OPC UA and feed data to it as desired for public exposure.