Hi, Thanks for your question. This is correct, the end of expansion was that date. I don’t have further information about why. Let us know if there is anything else we can help you with.
Dears, We have unity 300 Storage that have only 11 Disks installed and and it works fine but I need to increase the size of storage.  Since it have about 13 empty place to add more disks, I have contact a local reseller partner to buy more disks but he response me that this no longer available an say that  "Expansion for all Unity xOO/x50F models are no longer as of the September 30, 2022 End of Expansion date" so is it right that I can't buy more disks to install it in the storage. 
Buy More Disks for Dell Unity 300