It seems like the microbiome in your gut is really important and fiber seems to be key in maintaining it.
(This is not the advice I would give for a person who is already suffering from IBS, but as a regular human, I'd take care to not let my fiber lapse for too long.)
My current favorite healthy high-fiber food item is something we've been calling "Mashed root vegetables" with parsnips, and now also chia seed pudding (on the Congee (and related Chinese medicine/energetics) page).
Lots of other foods are good: blueberries, dried cranberries (TJs has organic ones without sugar), raspberries, broccoli, potatoes, peas, quinoa, etc. I know legumes are generally high-fiber, but are frowned upon by the Perfect Health Diet folks; I think red lentils might be ok if cooked properly, but their toxic skins have been removed along with a lot of the fiber :/ (speaking of which, I’ve been enjoying Amy’s quinoa, red lentil, and kale soup this winter—kinda in the direction of chili, but only 4 or 5g of protein per can. 🥣 )
The Best Gut Health Cookbook has a chart with foods and their fiber amounts. (That’s also where the Mashed root vegetables recipe comes from.)