Microsoft update the Product Terms site for May 1, 2021 by confirming that User SLs are not required for any user to join a Teams meeting or a Teams Live event. User SLs are also not required for external users to participate in Teams channels as a Guest with an Azure AD External Identity: https://bit.ly/3eGET3e.
Since September 2017 there have been two options for customers buying Windows 10 Enterprise E3 User SLs in CSP: a Local Only SKU which (not surprisingly) allowed just local installation of Windows 10 Enterprise E3, and a full SKU which included the additional rights to run Windows 10 Enterprise E3 in a hosted virtual desktop. From 1 October, 2018 the Local Only SKU will be retired and the price of the full SKU will be reduced to an ERP of $7 per user per month to match the previous Local Only SKU price. Find the announcement and a link to an FAQ here: http://bit.ly/2NmAemn.