It can't be just me but the quality of the particles seems to be different compared to previous versions. In Thaumcraft 5 (bottom image), you can see how the wisps don't look as faded and there are no weird borders on the textures like in 1.12.2 (top image).

Since wisps are full-fledged entities, this is not related to a particle system. Instead, I directly compared the codebases for the entity renderers:

In Thaumcraft 5, the aura node rendering is used additionally for wisps which makes sense when you compare the images. A design choice, really.

What's confusing about the wisps is that they use the particle texture for some of their rendering, but as ACGaming mentioned it is rendered directly and not via the particle system. I probably have to look into wisp rendering anyway because I thought they had an issue where they visually lagged behind their actual hitbox, but it's been a while since I checked.

Eldritch Guardians projectiles lack particles which they had in previous versions of the mod