I really don't get why whether "vassal feeding" is historical or not is important. Sure it may not be historical, but everything else you do in the game is not historical the moment you unpause. Arguing from history is rather futile: the real life has a really grey area between vassalage and alliance, and most historical atlases would simply include the purely nominally independent realms as part of the larger empire. I'm not that interested in the EU4 period history, but the gray area existed from the antiquity to now. When did the Italian and Greek city states ceased to be Roman allies and became their vassals, and became part of the Empire? Modern South Korea is an independent nation and is allied to the US, but South Korea is unable to declare war without the American consent. Is South Korea a "vassal" to the US? More in the EU4 period history, how about the local rulers in the Ottoman Empire? How autonomous do they need to be to be "vassals" or just "provinces"? The game can't simulate the gray area, and draws the clear-cut between an independent and a subject nation. At least that's my two cents, I'm not going to argue from history.
Whether vassal feeding is an exploit or an intended way to play the game at all would be a more legit question. For me, it's an intended way to play the game, because I've seen AIs doing the same to me. I got fed as a vassal and I was offered to be a vassal, so the AI knows the strategy, though they may not be able to do it as extensively as some humans who almost entirely rely on it - the question is how much is too much. And it's completely up to the players: if you want to roleplay a historical drama, you probably won't like seeing people painting the whole continent with the same color. If you're a wannabe Genghis then you'd jump on it. The option is there, the AIs know how to do it but prefers to keep it as a secondary strategy, if the human decides to use it as a primary mean or not is up to the human. Whether you want to be a roleplayer or min/maxer is your choice, and who has the rights to say one is inherently better or worse?
There were no any real life vassal feedings. People say it is a feature but even eu4 history files misses it. If you look different starting dates there are close to zero countries who have vassals and even less situations where they could be fed.
I haven't bothered checking EVERY historical diplomatic situations, but Khmer is one example where a vassal continues to exist throughout the in-game period. It begins as an Ayutthayan vassal, but it changes hands between Ayutthaya and Annam (a southern Vietnamese revolt state) back and forth with few brief periods of independence in between. If anything is ahistorical, it's that the in-game Ayutthaya would annex Khmer as soon as they can rather than keeping it as a vassal.
That said, returning cores to vassals is something that the AI doesn't do as often as they could. I must admit this. France never demands England to return core Labourd to Armagnac (I tried as Armagnac three times, sieged my core, France never gave it to me) and I am yet to see the AI Ayutthaya demanding Lan Xang to return Khmer core in Stung Treng. But then again, Labourd is a French core as well, and Ayutthaya usually doesn't go to war with Lan Xang right away because Aceh is the bigger enemy and I can understand the AI decisions in both of these cases.
But it doesn't mean the AI doesn't know how to return cores to its vassal. I wish I had taken a sceenshot, but I've seen France demanding Aragon to return core Roussillon to its vassal Foix. The AI is capable of it, and will do it if there's an opportunity and a reason to do so. It's just not their primary strategy, which is sort of understandable as a gameplay design since: if all AIs decide to go vassal feeding, the alliance and diplomacy system would break down, the blobs will form much quicker, and people will complain even more.
So, in the end, if you want to use vassal expansion as your primary mean, or only as a secondary mean, or never to use it at all, is your choice. For the AIs it's clearly a secondary mean, but they know how to do it.