The requirements are well documented by third-party teardown, and dozens of companies make chargers that include the necessary pull-up resistors. So as the GP said, Apple is doing nothing to prevent third-party chargers, and apart from the existence of the cable authentication, is doing nothing to prevent third-party cables, either.
The problem is that there seems to be a strong correlation between willingness to pretend that your products are genuine Apple products and willingness to cut corners in the design that result in dangerous products. Legitimate third-party chargers from known brands generally work very well. Fake chargers that try to look like Apple products are a different story. It is legitimately hard to squeeze the necessary electronics into such a small package, much less to do so safely. As a result, Apple knock-offs tend to be significantly less safe than chargers made by people who aren't trying to pass their products off as Apple hardware.
And the knock-off fake Apple cables tend to be low-quality junk that fails after a couple of weeks of light use, unlike more legitimate third-party cables (e.g. Amazon Basics), which tend to be at least as reliable as Apple's cables, if not more so.