fiber has had absolutely the opposite effect for me on flatulence.
Even when the probiotics contain actual living bacterial cells, and even if those are bacterial types that actually have a benefit, eating bacteria is a terrible way to get them into your gut. Thats part of the purpose of gastric fluids. Hydrochloric acid isnt conducive to bacteria surviving the trip. AFAIC the whole industry is snake oil.
You can deliver bacteria to the gut but the entry point is from the other direction. And who the fuck wants that?
Eating things that promote good bacteria (like lots of fiber) and not eating things that fuck them up (like lots of sugar or artificial sweeteners) is the way to go IMO.
In any case, I don't think most people have digestive issues because of their gut microbiome. They have digestive issues cause they eat like 1/10 of the amount of fiber they need for normal digestive function.