I’m not sure if it is blocked, per se. According to the Internal defects fixes list, Defect 188917 is resolved with
Information that MSSQL 2016 database and later is not supported by BigFix Inventory
This corresponds with the warning message in the Fixlet itself, about SQL Server 2016 being unsupported. Also, based on my admittedly cursory examination of the contents of the BFI-server-220.127.116.11-20180620-2309-windows-x86_64.zip file, there don’t appear to be any tests for SQL Server compatibility or version levels.
It would appear from the information provided by Microsoft that this change in datetime2() behavior only applies to compatibility level 130 and above. So keeping your database at SQL Server 2014 compatibility seems like it would work.
I would be concerned about getting into an unsupported configuration if you go down this road. That being said, the following from the Knowledge Center seems like good advice:
It is strongly recommended, especially in big environments, to first perform the upgrade in the test environment. To do this, back up your production database, restore it on the test server, and perform the upgrade there. If it is successful, perform the upgrade on the production server.
Then run the upgrade in interactive mode, rather than from a fixlet.