Old servos

That tiny capacitor is a confidence booster, rather than a practical idea! In the event of a power failure, it MIGHT run the receiver for one second, but if a servo move had been commanded, it would drop the stored charge instantly.
Try it and see, plug it into a spare channel, fire up the tx and rx, wiggle the sticks, then switch off the rx, and keep wiggling the tx sticks!
To have a useful stored charge, you'd have a cap the size of a tennis ball, only tubular!

For whatever reason - brown-out's only seem to affect early Spektrum receivers, anyway.
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In case you aren't sure what a brown-out is - it is where, due to the load on a battery, (all servos moving at once, etc) the current being drawn causes the voltage to dip momentarily, to the point where the receiver temporarily stops working. This was particularly noticeable as the pack charge reduced through usage during the day and was a good indicator of time to pack up and go home!
Models with the battery as a separate pack running the radio (i.e. an i/c model) were more prone to this, than an electric-powered one with a BEC and a much larger output-capable battery.
With most electric models, the speed controller would shut the motor down before this low voltage level was reached, so is doubly 'fail-safe'. The receiver, of course, would continue to function normally for a significant time after motor shut-down, leaving plenty of time for even a floaty glider-type model to be brought down safely under full control, just with no motor power!
Again, most speedo's these days will give you another 'squirt' if you close the throttle for a few moments, then re-open it, should the model be dropping short of the field.

Eric said, Again, most speedo's these days will give you another 'squirt' if you close the throttle for a few moments, then re-open it, should the model be dropping short of the field.
Just try doing it when the models dropping behind the trees into the turbulence. You know you should close but do you? I know I can't! Ask me how I know!