did you actually bother to check? do you have an actual account to login to https://www.dell.com/support/home as not all resources might be openly available?

for example the DD300 product page https://www.dell.com/support/home/en-us/product-support/product/data-domain-3300/overview states no EOSS date yet, but points to a general Dell "View end-of-life documents" KB article: https://www.dell.com/support/kbdoc/en-us/000185734/all-dell-emc-end-of-life-documents?lang=en which then points to various excel sheets:

https://supportkb.dell.com/attachment/ka06P000000PkE5QAK/EMCSoftwareReleaseandEndofServiceLifeNotifications_pkb_en_US_1.xlsx

https://supportkb.dell.com/attachment/ka06P000000PkE5QAK/EMCHardwareReleaseandEndofServiceLifeNotifications_pkb_en_US_1.xlsx

and :

https://supportkb.dell.com/attachment/ka06P000000PkE5QAK/EMCOperating_EnvironmentReleaseandEndofServiceLifeNotifications_pkb_en_US_1.xlsx