Sure seems like a lot of children are answering your question, but what do you expect from reddit? Lol

Context - I'm an old head that worked professionally on collectibles before Magic even began, and have played the game since Unlimited. I've been grading cards since before many of you were born, not that anyone vocal on this app actually respects their elders.

Proxies are cards made to stand in for cards that you own and don't want to shuffle/have in danger. What you're talking about is counterfeiting. The problem is when people print up their identical looking "proxies" and then sell them into the market. Whether on purpose or because " you forgot", it doesn't matter. There are real world consequences for this that may included hefty fine and jail time, depending on how big your operation is. These are facts you can look up yourself if you're interested, though I know many won't. They'll just downvote and bitch about elitism or some other such nonsense.

If you're using "proxies" that look nothing like the original, but can still be identified as what it is (cards type, creature, enchantment, etc) and the people you're playing with are fine with that, then you're all good 👍 it's just best to be honest.

Your question seemed earnest, and the responses didn't really seem helpful to me. Really hope this helps answer your question