If you are one of the multitude of users/developers who depend on MicrosoftAjax.js via accessing it in the _layouts/15 or _layouts/ directory and found that it is now gone from your SharePoint Online tenant or local SharePoint instance, you are not alone. Microsoft deemed this unnecessary as they believed it was not in use except for legacy Excel needs. In reality, many of us depend on it for client side operations.
After conversations with Microsoft this week, they stated that a KB Article and solution will be posted in the near future but that using the CDN for MicrosoftAjax.js is acceptable. I would recommend downloading the script from the CDN and referencing it locally in case of CDN issues. The CDN is located here: https://ajax.aspnetcdn.com/ajax/4.0/1/MicrosoftAjax.js
UPDATE: After Microsoft worked internally and realized the impact, they have begun rolling back this change and restored MicrosoftAjax.js to tenants. Check to ensure yours is restored. If not, use the method above.