Equalizers with low data rate systems
In the $2.4 \textrm{ GHz}$ band, the data rate is indeed just $250\textrm{ kbit/s}$ but the direct-sequence spread spectrum (DSSS) coding results in a bandwidth of $2\textrm{ MHz}$. This means that there will be flat fading only if the channel's coherence bandwidth is significantly greater than $2\textrm{ MHz}$, say around $10\textrm{ MHz}$ (this is just a fuzzy estimate). This corresponds to a maximum delay spread of approximately $1/(10\textrm{ MHz})=100\textrm{ ns}$.

Consequently, any environment with a delay spread greater than approximately $100\textrm{ ns}$ will result in frequency-selective fading. So in relatively small indoor areas the fading will be mostly flat, but in large office spaces or outdoors you can expect frequency-selective fading.