I don't remember every argument in there but it seemed that there are good reasons not to add it unlike a NonZero integer type which seems to have no real downsides.
From this, it follows that the right-hand side ("value expression", roughly equal to an rvalue) is not moved into a place called _, but is effectively just a value matched against a wildcard pattern. This does nothing (the value is not moved into any other place), so it gets dropped.
Yes, a YouTuber named EposVox released a video on AV1 hardware encoding when the first Intel dGPUs with support for it released: https://www.youtube.com/watch?v=ctbTTRoqZsM
Later on in the video, there are some graphs comparing Intel's AV1 encoder to SVT-AV1 at different speed presets. Even one of the faster presets (9) will comfortably stay above AV1 quality according to VMAF, and if you don't need real-time speeds you can lower the preset to get further ahead of the hardware encoder. (BTW: That video is >1 year old now, and SVT-AV1 had some significant updates in the meantime too. So the software side is probably looking better now.)
There's a feature that allows you to try and target a specific VMAF score, but it doesn't work very well. What av1an does do for you is increase the speed by a lot, especially for aomenc and rav1e, which don't scale well with high thread counts. SVT-AV1 doesn't really have this problem.
A faster encode might allow using a slower speed/preset setting, which would increase quality per bitrate a little bit. But I don't really consider av1an a tool that increases encode quality, to be honest.
I don't remember every argument in there but it seemed that there are good reasons not to add it unlike a NonZero integer type which seems to have no real downsides.