CM
In the literature: open
Where does 1/f noise stop at low frequency?
In plain words
If the 1/f law held down to zero frequency, the total noise power would grow without limit. No measurement has found where the law stops.
Precise statement
The integral of a $1/f$ spectrum diverges logarithmically at both ends, so a physical system needs a lower cutoff $f_{\min}$, set in the fluctuator picture by the longest relaxation time $\tau_{\max} \sim \tau_0 \operatorname{exp}(E_{\max}/T)$. Measurements over many decades have not found a flattening. Determine $f_{\min}$ in a specific well-characterized system (metal film or SQUID) and relate it to the fluctuator distribution.
What would settle it
Long, drift-controlled noise records on a stable device showing a flattening of S(f), compared with an independently measured distribution of fluctuator relaxation times.