They started opening the observation floor around mid 2019. You must register to get invited. I registered around August 2019 and didn't get my invite until Feb 2020. But it was a spectacular view!!!
The observation deck isn't open to the public, unless you are a Salesforce employee or one of their employees invite you.
I’ve been told it’s invite-only and will remain that way, unless you get express permission beforehand.
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