Exactol, FubarFX and I did a collaborative effort back in like, 2015 or something? We made pd_snowville, a PD version of Sawmill with some stuff added to it, like extra routes and buildings to accommodate the game mode change. What we did is setup a Github folder where we worked in and then divided the map up into instances so that each person had his own section of the map to play in as you couldn't change the same file at once (meaning that if I changed a thing in my section and one of my colleagues did that too in my section only one change could be made final as we now had two different versions of the same section). We then used Slack to get the communication part done and we did actually make the map, and the workflow worked quite well (other than Hammer being a massive pain in the rear, like showing stuff unhidden but have them hidden in code anyway resulting in leaks left and right). I mean it was nowhere near streamlined but it worked.
Remember, Hammer is old as balls, as is the entire Source engine. It was never designed for collaborative mapping. Maybe in the next iteration of Hammer something like that is build in, who knows. Source 2 is still as real as a narwhal at this point (aka we know it's there, we've seen some stuff of it but all in very limited numbers), but hey, you can always throw the suggestion to Valve.