This might be a bit of complicated question - but here we go. In SharePoint 365 they have depricated the asmx API sets as per this artical here.
https://learn.microsoft.com/en-us/sharepoint/dev/general-development/choose-the-right-api-set-in-sharepoint
We at this moment in time are using SharePoint 2016 and the high trust csom model - but need to use the OfficialFile / SubmitFile method in order to transmit files from a drafts site into our records. (using _vti_bin/OfficialFile.asmx)
Like I say this has now been depricated and I'm looking to redevelop our solution in 365. I'm wanting to do the right thing and just used csom to create the file in the drop off area. Then add in a flow to do what I want with it (I'm converting to pdf).
From the looks of things the routing rule is also on its way out as with the asmx and I should be using the flow to do whatever automated function I need.
Have I got the right end of the stick there with 'the plan' that microsoft seem to have laid out or am I missing something?