Made a tomato sauce in a skillet and everyone told me off: is the acid rule real or repeated folklore?

The rule is real, badly stated, and has a condition attached that almost nobody includes. Nothing you did was damaging.

What acid actually does. Seasoning is a layer of polymerised oil bonded to the iron. Acid attacks that layer slowly, and once it is thinned or breached, the acid reaches the iron itself and dissolves a little of it. That is what people mean by damage.

The conditions that matter, in order:

Time. A quick sauce of ten or fifteen minutes is essentially a non-event. Hours of simmering is a different proposition. Forty minutes is in the fine range.

How good the seasoning is. A thick well-built layer resists this for a long time. A new or patchy pan is exposed immediately, which is why the same dish ruins one person's pan and not another's, and why beginners get burned by the advice being absolute.

Leaving it in the pan afterwards. This is the real one, and the one the rule should be about. Simmering for forty minutes is fine; leaving the sauce in the pan overnight is where pans get stripped and food picks up metal. Cook, then get it out.

What damage looks like if it happens: a dull grey patch where the seasoning has gone, and a distinct metallic taste in the food. You saw neither, so you are fine.

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.

The distinction that finally made this make sense to me: there is a ruined pan and there is a ruined dinner, and the rule conflates them.

A ruined dinner is what most people actually experience. Iron dissolves into the food, and the food tastes metallic and sometimes goes an unappealing grey-brown. It is unpleasant and it is over, the pan is fine after a re-season. That is the outcome the rule is really protecting against, and it is a dinner-scale problem, not an equipment-scale one.

A genuinely ruined pan takes sustained abuse and is quite hard to achieve. Pitting from long-term neglect and rust does far more damage than any tomato sauce.

So the honest version of the advice: acid takes seasoning off faster than anything else you will cook, and seasoning is easy to put back. Know it is happening, keep the contact time short, do not store food in the pan, and re-season when the surface starts looking dull rather than dark.

One last practical note since you have a well-used pan: the more you cook fatty things in it, the more resilient it becomes to this. The pans that survive acidic cooking are the ones that get used constantly for everything else. Regular use is the actual maintenance.