Availability groups using multiple subnets changes
... but we are looking at modifying the RegisterAllProviderIP cluster resource setting to 0 due to some limitations on the MultiSubnetFailover parameter ...


Could you let me know what those limitations are? AFAIK there aren't any... MultiSubnetFailover is the defacto way to deal with your issue. Period. Hence why I would be interested to know what the limitations of the parameter are.


  My question though is if AlwaysOn changed the way connections are made in any of the newer versions of SQL (or .NET Framework) that prevents the need to do some of this.


No, there are no changes in SQL Server. There is a change in .NET 4.6.1 that will use multisubnetfailover logic if the initial connection doesn't happen in the first 500 miliseconds if multisubnetfailover was not added to the connection string.


  Is “MultiSubnetFailover=True” or changing the RegisterAllProviderIP still required in SQL 2016 or is any of this handled automatically\different?


You should ALWAYS use multisubnetfailover=true unless the client driver does not support it... in which case, upgrade the client driver and/or application... or deal with the application timing out.