There was an attempt to provide the switch port data for MXs a few years ago. It only lasted on the dashboard for a couple of days and never made it to the APIs due to numbering errors that were never resolved. An example of this is the MX75 where ports 1-3 are WAN ports so port 4 is actually called LAN port 0 in the back end, with port 5 being LAN 1 etc.
I don't know why they didn't fix it and republish the page as it doesn't seem an insurmountable issue to me.