I found that the quintessence tag exists. For convenience, here is the tag excerpt:
In Mage: the Ascension, quintessence is the substance that makes a thing "real" or "actual."
And tag wiki:
In Mage: the Ascension, quintessence is the substance that makes a thing "real" or "actual." It's manipulated through the Prime Sphere. Mages store it within their Patterns and spend it to augment spells or create magickal effects. These questions relate to the collection, use, and lore behind this substance.
At the time of writing this, nine questions are tagged with it.
The tag wiki is entirely correct: quintessence is a term with associated mechanics in Mage: The Ascension (MtAs) game which is part of the old/classic World of Darkness lineup. However, I hardly see how it is relevant to have its own tag. It only applies to MtAs games and there is not much knowledge that is related directly to quintessence that is not just knowledge of the lore and mechanics in MtAs games.
I wonder - is this tag really needed?