Encryption isn't the same as hashing. Encryption is two-way.
The previous comment did make the encryption / hash distinction - though I can totally understand how his post might have been misread that he was recommending the same mechanisms for both sets of data.
OK, so slack stores a username, name and email address for each user. This is visible to everyone else in the same Slack team at minimum. You also need it for e.g. password resets, perhaps billing.
We can assume they aren't total idiots and there's a Internet facing application server that connects to a internal-only database server that has this data. Also, assume SQL injection is not the attack vector.
How would you apply encryption to protect the username, name and email from an attacker that has gained access to the application server? I've gained some shell on the server and have 24 hours minutes to extract data. I can see all the files on the server but maybe as non-root but just the user that runs the application. How can you, as a security sensitive application developer, stop me if I've gotten so far?
What could you do with a one-way encrypted phone number? I'm not able to enter a phone hash to make a call.