Interesting question. It used to be quite common practice to fit them, back when forks were spindly affairs that 'walked' like someone with a dildo stuck up their arse. But then manufacturers started to make forks thicker, and to incorporate sturdier braces under the front mudguard, and they seem to have fallen out of popularity. USD forks killed them altogether I suppose.
As I mentioned elsewhere, fitting an R6 shock to mine, enabling me to push the bike harder, seemed to highlight some flex in the forks - often the case that if you improve one area on a bike, it'll show up the limitations of something else. In which case, a fork brace would once have been the logical choice for a next step.
Two kinds of fork braces (or used to be): The Micron/Telefix type that clamped around the lower fork. Some claimed that this could cause 'stiction' in the fork action, where the stanchion didn't slide smoothly in the fork lower, presumably because it distorted the lower fork in cross-section. Then there was the 'loop' type that bolted to the mudguard mounts and looped over the top of the mudguard - the preferable method for 'purists', avoiding the stiction problem.
These are just thoughts - I might conceivably be talking bollocks
