In 2.7.x it's usefull to pan only in horizontal or vertical, which I use a lot. I searched everywhere and didn't figure out, it doesn't work anymore? This page here https://developer.blender.org/T55194 says just "NO REPLACEMENT".
I solved the problem manually add key map item in the preferences.
User preferences - Input - 3D View - 3D View Global - Add new - in field "Identifier operator to call on input event" type view3d.view_pan - then choose your key and action Pan left or right \ up or down.
This is not solved by Import key configuration from blender 2.79.
I can't comment so here is an answer:
Fairly sure this is not possible in 2.8 for the moment. This requires specific RNA (?) in the hotkey bind (you can see this in 2.79 when you search using hotkey and for "shift scrollwheel" where there is this unique RNA and "panning" tab on the hotkey you can't get by creating new hotkey) Since there is no default hotkey entry for this in 2.8 I don't think you can make this without coding. (I tweeted at Pablo, though, so if he happens to reply with solution I'll update here)