This question is based on Astrophysicist Thanu Padmanabhan's online lecture here, time around 13.00-14.00.
Consider a map where the absolute time $t$ is mapped to another absolute time $f(t)$ i.e., $$t\to \bar{t}=f(t)\tag{1}$$ where $f(t)$ is a single-valued, monotonically increasing function of time. For example, $f(t)=at^3$. Under this transformation, the Newton's law $$m\frac{d^2\textbf{r}}{dt^2}=\textbf{F}$$ changes to $$m\frac{d^2\textbf{r}}{d\bar{t}^2}=\alpha(t)\textbf{F}+\beta(t)\frac{d\textbf{r}}{dt}\tag{2}$$ where $\alpha(t),\beta(t)$ can be expressed in terms of $f(t)$ and/or its derivatives.
If I understand it correct, then he says such a transformation is not allowed because even if one starts with $\textbf{F}=0$, after one makes the time-transformation (1), the particle is no longer force-free due to the second term in (2) on the RHS. It looks like a "viscous drag".
But the real reason such a transformation is not allowed, I think, is that it makes the flow of time non-uniform. As time passes, the time flows faster and faster. Why should we focus at what happens to Newton's law under $t\to f(t)$ when such a time transformation is also not possible in special relativity. What is the significance of his mathematical exercise?