Adding my support to this as advised by HubSpot Support. This functionality was present in the Desktop Add-in, now support for this has ended we have to use O365 add-in moving forward and users are frustrated by having to have this pane open at all times if they want to log emails, very disappointing that functionality has been lost due to moving to the supported solution.
Please note, Office 365 was rebranded to Microsoft 365 in 2020, all the documentation still references O365.