Would cleaning FTC_FleetHistory make a noticeable difference ? What exactly is stored in it ?
To answer your latter question, in-game each and every fleet has a history of the actions it's taken dating back to the initial formation of that fleet. So a cargo fleet might have records of picking up mines on Earth, unloading them on Luna, etc. Deleting the entries in FCT_FleetHistory removes all those records. There's no gameplay effect, but if you RP a lot or want to know what a particular fleet did at a particular time it would remove that information.