Background
I have three old hosts running ESXi 5.1 U3. According to the compatibility guide, newer versions of ESXi are not supported on this hardware. Later in the year new hardware will be purchased and a current a ESXi version deployed. I'm running VCSA 6.0.
Requirement
I have an immediate need for some external storage. I would like to deploy an iSCSI device.
Issues
Questions
Regarding an "old" link to an HCL...that doesn't exist (to my knowledge). You're so far out of support at this point that it was aged out literally years ago.
Regarding the iSCSI support, there's really not such thing as a "generic" iSCSI device as they all behave somewhat to very differently even if the protocol they implement is said to be the same. I think at this stage, owing to the fact that you are running such an out-dated version of ESXi (nevermind supported), you'll just have to try it and see. It'll probably "work" to some degree, just don't pick new hardware on which to implement the iSCSI target.
Regarding an "old" link to an HCL...that doesn't exist (to my knowledge). You're so far out of support at this point that it was aged out literally years ago.
I was able to find ESXi 5.1 in the VMware Compatibility Guide from December 3, 2018. Unfortunately, I cannot use that link to retrieve any information so I'm still no further ahead.