This isn't a mod, per se, nor is it a lowering of the game's actual settings. Instead, it's the result of tweaks to the game's texture files, which basically remove everything except the bare necessities.
It's like a more colourful Tron. I love how minimal the "GOAL" explosion is, too. Below is the full video showing a variety of effects and maps looking terrible/amazing.
If this doesn't start in the right spot, head to 4:53.
To try this out yourself (via By ２ ０ ９ ４ ＿ ♥),
1. Go to your Rocket League installation folder. (Default: C:\Program Files (x86)\Steam\steamapps\common\rocketleague\)
2. Now open the "TAGame" folder and the "Config" folder.
3. Spot the "DefaultSystemSettings.ini".
4. Create a backup of the "DefaultSystemSettings.ini", by copying it and rename it to "DefaultSystemSettingsBACKUP.ini" (or something like that).
5. Now open your original "DefaultSystemSettings.ini" with any texteditor and replace the content with THIS. The file has to be called "DefaultSystemSettings.ini".