It looks like TruckersMP are making progress.
August 14th 2020, 8:21 pm | George Grayburn
We've all been waiting nearly a month now for the new update to come to TruckersMP. However, understandably, there have been some hurdles and barriers for the developers.
Today they released a development update stating that "FMOD is still a major factor towards the difficulty in updating multiplayer, as they present further back-end changes." They have also said that they are anticipating a release towards the end of August or early September.
TruckersMP have also said why updates are taking so long. According to their blog post, only their lead developer, mwl4, has the knowledge to port from 1.37 to 1.38 and he is working on the update along with other personal commitments.
If you want to play multiplayer, you will need to downgrade to 1.37. You can do that here.
You can find more info on the 1.38 update here