I have studied effects of bend_smooth, and I share some results.
In SFZ specs, smoothcc under Modulators is a "see also" to opcode bend_smooth.

The experiment is to load a sine tone and modulate midi pitch bend with a ramp lfo.
The results were observed on spectrogram in real-time.
Then, it was matched to a faust program which plays a sine modulated with identical LFO and range.

• sfz: sine440.sfz.gz
• pitch modulator jack client: bendlfo.cc.gz
• sine modulator sinemod.dsp.gz

⚠️ note that the smooth filter must be applied on the CC values, not cents values which are result of the conversion following.

The smoothing filter is one-pole LPF which is defined by a time-constant tau. (cf. faust basics.lib)
pole = exp(-1.0/(tau*Fs))

These are approximate results measured visually.
tau is observed starting at 0, varying +3 ms by bend_smooth unit.

#bend_smooth        tau measured (ms)
10                 30
20                 70
30                 90
40                 120
50                 160
60                 180
70                 220
80                 230
90                 270
100                300