I'm not familiar with Storwize, but the lat time when I disconnected shared storage from an ESXi host for an upgrade was in the ESX 3.0 times.
Does IBM recommend this, or what's the reason why you are thinking of this?
This was recommended in 3.0 time because you can easily select a already attached LUN with existing VMs on and choose it as a install target for ESX.
We use SAS Shared storage for 7 years now (Powervauld MD32x0, SCv2020 and up) and the ESXi upgrade process is different compared to the old days.
in the document
Bestt practices to install or upgrade to ESXi 6.0 (2109712)
i read this
f a Fiber Channel SAN is connected to the host, detach the fiber connections before continuing with the upgrade or migration. Do not disable HBA cards in the BIOS.
Only for fibre channel san?
if you upgrade through VUM you have not manually to select anything.... for me there is no need to detach the storage when applying patches or upgrades this way.
If you manually boots from a ISO to upgrade you have to select the right things.... and if its not your best day you may choose the wrong option. If you feel better follow the vendor recommendation.
Note: If youre late with reconnecting your SAN back to the hosts and ESXi finished boots you may want to check your adv. settings for the ScratchDisk because it will be revert back to default when the real target isnt available.