This is the most glaring exception to this site's policy of hosting the original version of files whenever possible. Videos which are MP4 at the source are still required to go through a lossy conversion process in order to be posted here.

This notably affects Twitter as well as a number of video hosting websites, but sometimes the artist will make the source video directly downloadable from a file host such as Google Drive, and it's still MP4 so can't be posted. For an example, see the description of post #3046787. Remember when one of e621's selling points was that our files were higher quality than FurAffinity?

The closest thing to a justification I found was this 19-month-old post:

mairo said:
MP4 format is not currently supported on the site, you can check on earlier mentioned e621:supported_filetypes for supported filetypes as that page will be updated to reflect currently supported formats, current ones are Flash (swf), APNG, WebM (VP8/VP9) and GIF.
MP4 was not supported by Firefox back when site implemented WebM support and even now, MPEG LA licensing of h264 and h265 can apply on both website and software, where successor h265 is only supported by Safari right now and has much stricter terms. Hopefully the site at some point get h264 MP4 support, but right now we go by this and VP9/Opus WebM is highly preferred with these kind of conversions as it provides as good if better quality for bitrate compared to h265 MP4, but is completely free format which is widely supported (excluding Apple and Safari) and even used by big names like Youtube and Netflix for this reason.

But licensing can't be that much of a concern any more, because five months later, suddenly e621 is hosting MP4 files after all - just only if they were converted from WebM uploads. MP4 to WebM and back to MP4! Is an end to this farce in sight?

If I had to guess, it could be due to the 100Mb size limit.

Mp4 usually has better quality but tends to be bigger in size than webm, also, while mp4 is compatible with most if not all devices, webm was literally made for online streaming, and most browsers support it.

wat8548 said:
But licensing can't be that much of a concern any more, because five months later, suddenly e621 is hosting MP4 files after all - just only if they were converted from WebM uploads. MP4 to WebM and back to MP4! Is an end to this farce in sight?

This is basically the same way that derpibooru handles it (they only use WebM as well) that because these are samples rather than files themselves, if someone does come down knocking on the door with bill, these files can simply be deleted immidiately.
Now if we did accept h264 and h265 as codecs and this happened, now we would have a loooot of posts without any files. That would be bad.

What would be ideal is if artists also started to adapt to the situation and provide WebM exports and encodes directly instead of MP4. Because these formats are the lossy end user formats, so for artist it's simply a choise which to encode into and at which point quality of WebM should be higher.
Many artists do already do this as they use our website as primary place to host HD versions of their animations, so that is already nice to see. I have also been asking some artists if they could start utilizing APNG over GIF, which will also significantly improve the quality of the material in future.

We have upcoming post replacement system so this should ease the situation where we do start allowing MP4 uploads (turns out we do not count as "video streaming site" so license is free, codec licenses expire, someone starts paying the license, we get fancy hardware to transcode all filetypes so we can start accepting stuff like AVI files as well, etc. scenarios), so the previous transcodes can then be simply replaced (instead of causing similar situation to tumblr uploads), but that would maybe happen when the licensing expires for the format and with the latest revision of the format that would be around 2027.

azero said:
If I had to guess, it could be due to the 100Mb size limit.

Mp4 usually has better quality but tends to be bigger in size than webm, also, while mp4 is compatible with most if not all devices, webm was literally made for online streaming, and most browsers support it.

Most content fits under that filesize limit even as h264. Altough you are right that if there are situations where the video was encoded from lossless source format or from way above 100MB MP4 file, then the quality of VP9 upload in here should be the best it can be.
However another problem with filesize is that VP9 is equilevant in quality to h265, but h265 licensing and useage is even more bothersome from h264 which basically any artist sharing MP4 use, because even playback requires license unless you have it bundled with fully free software (this is why Windows players pop up to buy 2€ license as the players are part of OS which is commercial software, while VLC just plays it).