The only way to avoid that warning is make sure that the maximum key length of the index is below 1700 bytes. There is no setting to turn this off. After all, it is telling you that you have created an accident waiting to happen. Then again, if you know your data, you may be confident that it will not happen that all key columns are long, and in that case you can ignore the error.
I should add that I'm purely talking from an SQL Server perspective. I don't work with Azure Data Factory, so I can't talk to the impact of this warning in ADF.
I think you should fix the syntax errors. :-)