What is the DX scratch org feature for AccountContactRelation CustomObject

it's old question, but in summer19 released a new feature called --"features":["ContactsToMultipleAccounts"], we r using same feature for "Contacts to Multiple Accounts Settings"

Current they best solution for this at this time is using the OrgSource Pilot program. Obviously it still has some work but it's definitely a lot easier to work with then not having it. First step is to request and gain access to it. Once you run a quick test with the pilot org they provide you then you can request to have your production org enabled with the feature.

The steps are pretty easy. Essentially you need to create a shape (this is essentially what a definition file is, but this is being generated by your given org).

sfdx force:org:shape:create -u Production

Then once that completes you can see it in your list like so:

sfdx force:org:shape:list

Once this is complete you can create a scrath org definition json using the source org id of your shape:

{
    "orgName": "My Company",
    "sourceOrg": "00D1N000001Bn#####",
    "hasSampleData": false
}

Then create a new scratch org using this definition file:

sfdx force:org:create -f ./config/production-scratch-def.json -d 30 -s -a test

That's it. Doesn't solve every problem yet but it does solve some. I noticed right away that this still doesn't account for things like Forecasting... (at least it didn't at the time).