No, it isn’t. Nigella’s website, for instance, talks about ‘the most forgiving recipe for banana cake ever’, and there are plenty of other examples from both sides of the Atlantic. I don’t know how long it’s been around, though, and it would be interesting to track down an earliest citation. I asked two cooks in my family whether they knew the expression and neither did, so my feeling is that it’s a fairly recent usage.
I’m not surprised that it receives no separate mention in a dictionary, as dictionaries don’t provide a systematic guide to the collocations that belong to a particular meaning. And in the general sense of ‘easy’, ‘safe’, ‘comfortable’, forgiving has been used in a wide range of inanimate contexts – workplaces, enterprises, timetables, climates, surfaces, lights, clothing, and many other nouns have all been described in this way. Quite a common collocation is with piece: a forgiving piece of clothing / machinery / meat... So, as long as a dictionary illustrates from some of these, the broad sense will be covered.
A forgiving recipe, it seems, is one which does not require exact measurements, where some ingredients can be substituted without the result being affected, or where a cook can get it wrong and it still turns out OK. I'd have thought that, in the context of cooking, this usage has moved away sufficiently from the general sense to warrant its appearance as a separate dictionary sub-entry. A couple of dictionaries are already taking notice of it, and I don't think it'll be long before we see it in all of them.