I cook acidic things in iron most weeks, so here is the practical version of when it is fine.
Fine: a pan sauce deglazed with wine or vinegar. A tomato sauce for half an hour. Braising with tomatoes in an enamelled dutch oven, which is not bare iron at all and where none of this applies. Anything where the acid is one component among fat and other ingredients.
Push it and you will notice: a long simmer of something highly acidic in a bare pan: a thin tomato sauce for three hours, a vinegar-heavy reduction. Doable in a very well seasoned pan and it will take some of it off.
Do not: store acidic food in the pan, and do not cook long acidic things in a new or recently re-seasoned pan.
The practical tell during cooking: if the sauce starts tasting metallic or the colour goes slightly grey, stop and move it. That is the signal, it is unmistakable, and it happens long before anything permanent occurs.
And if you do thin the seasoning: it is not a broken pan. Dry it, wipe a very thin film of oil on, heat it past the smoke point, and you have repaired it. Seasoning is a consumable. Treating it as a precious finish that must never be touched is what makes people scared of their own cookware.