Despite ending support for IE11 June 22, it was still the default browser for addins until I believe Jabuary this year ???
Please note that the browsers used by the Office Add-ins are determined by the Operating System and Microsoft Office versions. To utilize the Speckle Excel add-in from your desktop, please update to a version of Microsoft Office that utilizes at least the first Edge browser.
Actually, I see it reports Version 1808, which is a volume installation version and also the latest build number so it should pick up Edge and not IE11 - so I’d discuss this with your IT department. There may be a reason to keep your versions pinned there for older addon compatibility, but supporting IE11 with Speckle Excel is not planned.