- Earlier the IBM TSM version 7.1 and before, did not support SnapDiff backup with Clustered Data ONTAP Systems. - IBM later released a Cdot supported version. Hence, after TSM 7.1.2, Cdot was supported for SnapDiff Backup. - Customer has to use IBM TSM 8.1.4 which is compatible with ONTAP 9.3. Refer
With regards to your query regarding "Is 7-mode already end of life?" as you want to "extend the life of the V3250 to June 2021."
Ontap 8.2.5 7-mode, which is supported on the V3250 has End of Full Support (EOES) on 31-Dec-2020, but the End of Limited Support (EOVS) continues till 31-Dec-2022. So you should able to continue using this Ontap version till June 2021.
Definition of End of Limited Support (EOVS): The time period where NetApp provides partial support for a version of a software product, including Technical support, root cause analysis, documentation and software available online. Service Updates, maintenance and patch releases are not provided for versions under limited support.