Reg. D 506(b) is the main securities law exception (it's been around a long while) and doesn't require companies to research whether someone's really accredited. Having the investor check a box is enough, and checking the box really does protect the company from getting sued by the investor, who'd have to 'fess up and say they lied. This rule, though (506(b)) doesn't allow for "general solicitation," i.e., for companies to advertise or solicit selling the deal to people they don't have a pre-existing relationship with.

The SEC / Congress changed that a few years ago when they adopted 506(c), which allows "general solicitation," and therefore advertising. 506(c) didn't replace 506(b), but is a new alternative. Under this new alternative, like I said, you can advertise, but there are 2 conditions: (i) while the advertising is deemed an offer to unaccrediteds, you can only actually close the deal with / sell to accredited investors, and (ii) you've got to do a reasonable amount of diligence to figure out if they're really accredited. This is how platforms developed allowing deals to get pitched to groups of accredited investors who had no relationship with the selling companies previously. The platforms asked their customers to prove they were accredited, by providing bank / tax statements.

So both are true: you can still do the traditional deal, and this is what I mostly see as a securities attorney for startup / venture backed companies: they don't advertise, and asking investors to check the box is enough (for now). But some companies want to advertise, and if they do, diligence is done and proof required. When the SEC adopted this approach for 506(c), a number of people thought that the SEC would eventually also ask companies to do some diligence in the 506(b) / no advertising context, too, but it hasn't happened, yet.