Microsoft announced that Microsoft Skype for Business Online will retire on July 31, 2021, as Microsoft Teams has become the core communications client for Microsoft Office 365.
Note: Neither Skype consumer nor Skype for Business Server will be affected by the retirement of the Skype for Business Online service. Current Skype for Business Online customers will experience no change in the service up to the retirement date.
To align with this schedule, support for the integration of third-party party Audio Conferencing Providers (ACP) into Skype for Business Online has been extended to July 31, 2021, with limited support for remaining active tenants to allow additional time for transition. This is an update in the ACP timeline that was originally announced back in April of 2018.
Please find the official announcement and some helpful guidance here: https://techcommunity.microsoft.com/t5/Microsoft-Teams-Blog/Skype-for-Business-Online-to-Be-Retired-in-2021/ba-p/777833.