Using New Outlook on Windows for personal email, calendar, and contact management
I apologize for the confusion. I use a Mac, and the MacOS Sequoia 15.1.1 version of Microsoft Outlook 365 calendar is where the 3 dots > Customize Toolbar are showing (see the first screenshot below) to add the signature option to the toolbar (Signature wasn't showing before I added it).
I also use a VDI with the Microsoft Windows 10 Enterprise (22H2) version of Microsoft 365. In that version of Outlook, the Insert > Signature feature is already present, and I didn't have to set it up to appear in the toolbar.
In summary...the Signature feature is indeed available, there just may be a conflict with how different IT departments are setting up the application builds. I hope this helps!