I have to avoid diet soft drinks, because in the US virtually all are sweetened with aspartame. There are controversial claims that aspartame can trigger seizures and migraines. I'm already an epileptic, and I do seem to have had seizures myself after drinking it.
But there can be a few hours onset delay, it doesn't happen the moment I put the glass down, and usually not at all, so the relationship even for me is not firmly established. Since I'm rather well regulated with medication, and very rarely have a seizure today (and then it's mainly if I should forget to take my meds), if I do have one within 24 hours of drinking a diet drink it makes me suspicious. Therefore to be prudent I simply don't have them.
And they say diet drinks do little for you anyway, because they may stimulate the appetite in other ways. You're avoiding sugar in the drink, but adding more calories from other sources.
When I want a soft drink I'll have regular 7-Up or Sprite, sometimes a root beer, none with caffeine. Also things like cranberry juice (good for the urinary tract), Gatorade or equivalent electrolyte, some orange juice, almond milk, original V-8 vegetable juice, and water with meals.