Updated July 20, 2022:
We’re resuming the rollout of this change in Current Channel. Based on our review of customer feedback, we’ve made updates to both our end user and our IT admin documentation...
Updated Jul 20, 2022
Version 9.0
Blog Post
MiFatina
Microsoft
Microsoftfastexcel thank you for reaching out, and my apologies on the delayed response.
The behavior of XLAM files with MOTW is not new; it was introduced in Summer 2016 as part of a security patch: https://docs.microsoft.com/archive/blogs/the_microsoft_excel_support_team_blog/excel-workbooks-may-not-open-after-installing-ms16-088
This behavior stems from the fact that Protected View allows users to view documents, not add-ins. Add-ins with MOTW will not open because there is no Protected View support.
Signing the VBA code in XLAM files is still beneficial: once the file is unblocked and if the publisher has already been trusted, the user will not be asked to "enable content." In addition, Excel will detect if the VBA code has been modified after it was signed, alerting the user that the file has been tampered with and is unsafe.
The Excel team is working on improving this experience by informing the user that the add-in cannot be loaded.
I'm happy to follow up with you over private message if that would be helpful.