I know that physicist do accept that fact as "an assumption", or "as a fact" due to "proofs" (or missing unroof).
But are electrons really to be considered as elementary particles?
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It only takes a minute to sign up.Sign up to join this community
Short answer: Yes.
Meaning: The behavior of electrons, the measurements from innumerable experiments, fit the mathematical model ,the standard model of particle physics.The particles in the table are mathematically point particles and the electron is one of them.
After all nobody has touched an electron , we know it exists from measurements, and to the best of physics knowledge those measurements fit a point elementary particle.
At this time the question about is the electron a composed particle has to be more a theoretical than a experimental contemplation. We haven't the tools or perhaps we are not willing to make such experiments. Fact is that your question seems not to interesting science at the moment. But the question is serious and very legitim. To accelerate an electron you need the interaction with photons. During positive acceleration the electron "consumes" the photons momentum (mostly partially and sometime full) and during negative acceleration it emits photons.
We know else that the electron has both a electric field and a magnetic dipole moment (a magnetic field) and that photons have a oscillating electric and an oscillating magnetic field. Furthermore the photons, accelerating the electron, shielding the electron's electric field. On the other side, during the interaction of an electron with the nucleus the electron emit photons and seems to stay without his electric field. It is not illegitimate to suppose that the electrons electric field nearby the nucleus is weakened. Have a look on my paper about composed particles and the one about magnetic dipole moments of the electron orbits.