So unfortunate news for you is it sounds like this closed source application uses a potentially deprecated feature of SQL Server if it's using
SAVE TRANSACTION, and was not designed to run for a database that's part of an Availability Group (or any kind of clustering for that matter). Even more unfortunate is your only three options are:
Thoroughly verify it isn't breaking anything when you use that functionality of the application. That should involve testing that functionality and measuring the results on your primary, and then failing over to your secondary and verifying the same results did in fact commit to that node as well, and the application is still working as expected on the secondary. If all's well, then just live with the error.
Reach out to the vendor and communicate to them that you're trying to run their application on a clustered server with an Availability Group and are receiving an error for a specific function of their application that appears to use a deprecated feature of SQL Server, and that you need their confirmation if this is ok, and/or does their application support Availability Groups. You'll probably want do this anyway, if you pick option 1.
Disable distributed transactions with
SET ( DTC_SUPPORT = NONE ); as you've seen this removes the error, but it does make your Availability Group less reliable which I'll answer directly to your questions below.
Out of these options, your best bet is #2, so that you can work with the vendor and let them know the full situation in which you're trying to use their application. It sounds like Availability Groups aren't directly supported by it and they'd be able to confirm that. Unfortunately for you, that's just a constraint from the vendor, and there's not much else you can do other than the options I provided above, and/or don't use Availability Groups / clustering, rather have an alternative high availability plan in place such as maintaining a completely decoupled, second server, which you regularly restore backups to. (Obviously this is less than ideal.)
To directly answer your questions on disabling distributed transactions:
What impact does this change have?
There is a risk of data loss from unhardened (uncommitted) transactions when a failover event occurs from the primary to the secondary. While this risk is potentially small, and I believe only affects your distributed transactions themselves (such as this application functionality with the
SAVE TRANSACTION code) it's still a risk. This is discussed in the Microsoft Books Online for Configure distributed transactions for an Always On availability group, specifically when they say:
SQL Server does not prevent distributed transactions for databases in an availability group - even when the availability group is not configured for distributed transactions. However when an availability group is not configured for distributed transactions, failover may not succeed in some situations. Specifically the new primary replica SQL Server instance may not be able to get the transaction outcome from DTC. To enable the SQL Server instance to get the outcome of in-doubt transactions from the DTC after failover, configure the availability group for distributed transactions.
Is the cluster still active?
Yes the cluster is still active and you should be able to easily test that by making a minor data change on the primary and watching it update on the secondary.
Is the data still replicated to the secondary server?
Yes, same answer as above.
Does it have a positive or negative impact on the performance?
Not relevant to the change, and probably measurably no difference.
Has a safety feature just been disabled?
Yes, your distributed transactions are at risk for data loss now if a failover were to occur when they were still active transactions (i.e. the secondary now becoming the primary wouldn't know if it's correct to rollback those transactions, rollforward or commit them), as discussed in my first answer to your questions. How much risk this is to you will depend on how important that data is for the feature that uses the
SAVE TRANSACTION code, and anywhere else you may be explicitly using distributed transactions.