Google has recently updated the help page related to Google Photos unlimited free storage feature. After this change, there seems to be a new limitation for unlimited storage of video files.
Until now, Google Photos allowed free uploads of photos and videos from your smartphone whenever you let Google compress your files a bit. In this way, the video files that you upload from your smartphone were compressed by Google and did not count towards your storage quota in Google Drive.
However, Google has limited what video file formats are compatible with this feature, which means that some video formats will now take up space in Google Drive.
The new policy came into force on December 6, 2018 and limits the supported video file formats to .mpg, .mod, .mmv, .tod, .wmv, .asf, .avi, .divx, .mov,. m4v, .3gp, .3g2, .mp4, .m2t, .m2ts, .mts and .mkv.
The reality is that this list covers almost all common video file formats nowadays , except for some weird ones like .vob or .raw.
Google may have made this change in response to people trying to take advantage of unlimited storage.
For example, users could camouflage a large file as an uncommon video format and then upload it from their phone to Google Photos. Since that file did not count for Google Drive, the user could store it in the cloud for free.