Well at least Slack doesn't even offer such stupid policies, so I prefer Slack. I began the deprecation process of SMS around 1999 with the first Blackberry which supported this awesome new thing called e-mail. E-mail is this FANTASTIC new thing that offers more than 140 characters, allows larger attachments. It works just like SMS, except you can receive them even if you change your SIM card while travelling! Your inbox moves with you from provider to provider! How COOL is that!
Seriously, nobody should ever require SMS in 2020. Kill that nonsense.
Well at least Slack doesn't even offer such stupid policies
It does (indirectly) if you use SAML single sign on (which many large corporations do). That is what is happening with Teams. You're not being asked to sign in to Teams itself. You're being asked to identify yourself via Microsoft single sign on and the people that set up that companies Microsoft single sign on have set SMS as the default 2FA.
Whether or not you should use SMS for 2FA is of course a separate discussion.
That's a company policy setting and not something inherent to Teams.