Tbh with how cheap USB chips are these days, you could easily house such an adapter inside the USB A plug. You’d probably lose charging (over 500mA) capabilities, though, because that sounds like something that needs expensive chips.

That said, for some USB cables, you can make a standards compliant cable. You will need a USB C cable with USB 3 capabilities (none of those cheap phone chargers) and no chip. Since the A-to-C cables distinguish themselves by putting resistors of certain values, a plug with the right resistors could downgrade a C-to-C cable with <60W charging capabilities to an A-to-C cable.

If you try to attach such a plug to a cable with a chip capable of negotiating voltages, you’d need some kind of intelligence to maintain feature parity, so that’s a good reason to avoid these things. I believe you can downgrade the cable to a 2.0 cable and retain basic functionality, but you’d sacrifice a lot in the process.

Unless you know the details about the USB and USB power delivery, these adapters are terrible, and that’s why u don’t think they should be sold to consumers. That said, for people who know what they’re doing, they’re fine-ish, though you’d be better off just buying a new cable.