The particle-physics tag (currently with approx. 3500 questions) is one of a few primary tags. Ideally all questions should be tagged with a primary tag.
The elementary-particles tag (currently with approx. 200 questions) is a secondary tag. It is mainly used for particle physics questions where it is important to distinguish between elementary particles and composite particles.
Example: Phys.SE often gets questions about whether the electron is an elementary particle or not.
For this role the tag elementary-particles does serve some purpose, and seems worth keeping.
A related secondary tag is the point-particles tag (currently with approx. 100 questions). Note that the notion of point-particles is used also outside the area of particle physics, e.g. in classical mechanics.