To my understanding, it's one of that usual warning Dell issued for BIOS upgrade/downgrade restrictions.  In this case, version 11 was a transition between these two CPU architectural.  it was a warning for users whom upgraded to a Broadwell CPU only.  Due to transition architectural of CPU, downgrading BIOS with a Broadwell will be restricted while a Haswell CPU should have no problem.
I currently have a T7910 running bios version A01 with two E5-2620 V3 processors.  I just ordered two E5-2690 V4 chips, and I understand I need to upgrade the bios to A34 from other threads.  I'm seeing a confusing (to me) note about bios upgrades from the Dell support page for this computer:  "BIOS version A11 and earlier releases are incompatible with Intel Xeon E5-16xx and 26xx-v4 CPUs (Broadwell). It is recommended that you downgrade BIOS to version A11 before downgrading from version A12 and later. This process is due to some significant changes required to enable v4 (Broadwell) CPUs. Also, along with that it enables Xeon E5-16XX and 26XX v3 (Haswell) CPUs and v4 (Broadwell) CPUs BIOS to stay in one BIOS stream. All versions of the Broadwell CPU enabled BIOS (A12+) understand how to migrate the Haswell settings. There are no upgrade restrictions." Do I need to do anything other than download and install the A34 version? Thank you, Curt Started running the new AI filters in Adobe Photoshop, and my old CPUs are running at 100 percent!
Dell Precision Workstation T7910 CPU and BIOS Upgrade