Similarly to how we’ve treated geolocation for a long time, the permissions change (crbug.com/780344) that enables notifications by default for the DSE has landed. This will roll out in M65.
This change enables the planned web rollout of a Google search feature that allows the user to get a notification containing their search results if they attempt to search while offline (implemented on the web via Background Sync \o/). Note, this feature already exists in GSA.
This change treats the user's chosen DSE as privileged, but that makes sense to me given how central the search feature is to a browser.
Please feel free to reach out if you have any questions.