quantum gravity - What areas of research question the existence of the Planck length?

As a follow up to What are the structures that exist at roughly the scale of the Planck length?, what areas of research question the idea of a Planck length (or Planck scale)? Or if there are no areas of research, what specific qualities make it questionable?

Answer

The Planck Length is equal to $1.616199 \times 10^{-35}$m which is roughly $100$ billion billionth of the diameter of a proton. It is a base unit defined in terms of the five physical constants $G, \hbar, c, (4 \pi \varepsilon_{0})^{-1}$ and $\kappa_{B}$.

No areas of current research (that I know of) question the idea of the Planck length or indeed any of the underlying physical constants listed above;