Found in routine PopeTech scanning. Appears to be a change introduced post-2.8.0. (From the scans, looks like it came in sometime between 7/24 and 7/31.) Should be easy, but needs to be corrected prior to next release.
So, rather than a specific change in ArchivesSpace, this was actually introduced via the Rails upgrade (5.0.x to 5.2.x) included in https://github.com/archivesspace/archivesspace/pull/1953/ . Previously, the image_tag method would fallback to using a filename as the source of automatically generated alt tags, however that behavior was removed in Rails 5.2 as it doesn't actually generate useful alt text in most cases. See: https://github.com/rails/rails/commit/9a74c7b99bf0ac901f86bb38372a68e157cf9c73 .
Moving forward we'll have to explicitly include alt text in _branding.html.erb and, likely more importantly, encourage others to do the same when overriding the branding template via a plugin.