If a store is found to have proxies on their premise the can loose their ability to sell WoTC products. So cards with "proxy" on them is bad. "Playtest Cards" would work, the issue is that there is no way of guaranteeing that it is a playtest card and not a real card with words written on it.
You could show up to a 15 proxy event and with 20 cards with "playtest card" written on them and say that 5 of them are real and just have "playtest card" written on them.
I don’t care if they have 20 proxies or 60 proxies as long as I can read and understand them from my side of the table. That is not always the case, and some are god awful. Hence why I advocate for counterfeits allowed as proxies to make it easier to distinguish and read them. Some people will hate that, and I understand why. I respect the store policy and follow it. If I’m lucky, I get to play once a quarter if ELD is hosting a tourney and I don’t have family responsibilities. It’s always fun and I get to reconnect with a few vintage players I’ve known for a long time. The more participants the merrier. There are decks I’m priced out of I’d like to play, but until the policy changes I will play what is legal under the stores policy. I recognize this opinion may be in the minority, and counterfeit cards are distasteful and wrong, but the pricing of cards has made this hobby unreachable for too many.