I have always used the deluxe product and have never had an issue downloading investment data. Now it says for 2021 that is only available with premier. Is that true?

I'm not sure what you mean by "investment data." The current list of features for TurboTax Deluxe Online says it will import from "financial institutions." I assume that means interest and dividends, on Forms 1099-INT and 1099-DIV. TurboTax Deluxe Online never handled investment sales, Form 1099-B.

Depends.  The Desktop CD/Download Deluxe can import all investment data. But in the Online versions you need Premier to enter investment SALES and gains/losses.  Online Deluxe can import Interest and dividends.  But the 1099s are not available yet to import.  Not until February and late Feb for some of them.    

Since Online isn't open yet are you using the Deluxe Desktop program?  It's probably just recommending you upgrade to Premier.  It's just a suggestion.  

If you use the ONLINE program you have always needed the Premier version to handle a 1099-B from the broker.

https://turbotax.intuit.com/personal-taxes/online/

ALL of the DOWNLOADED  versions can handle ALL of the same tax forms ... you get more "guidance/hand holding" in the Premier version but although it is "recommended" it is NOT required. 

https://turbotax.intuit.com/personal-taxes/compare/desktop/

READ the screens carefully to avoid the upgrade trap ...