I'm not HR. But the direct answer to "Is this the new professional attire not appropriate for interviews but once you get hired, it's okay?" depends on the specific company and the specific job. But, the practical considerations should center on attire to be worn to a job interview. Stick to what’s important to you, not your judgments on what is proper in general. Although I can tell you that for some of us low necklines (and short skirts) are distractions. Not such bad distractions actually, but certianly they should be avoided in a job interview.
Once you're hired, and are past whatever trial period is involved, it is harder to fire you and any workplace attire rules loosen up. Which I assume was why in your examples there was some variety.
And as one who has had to interview applicants for engineering posts (if that is applicable to you), just remember that your attire isn’t going to get you hired but it might work against you. Just wear something on the nice side of conservative, comfortable (especially if it may be a long day of multiple interviews, most companies will send you around to several interviewers), not shabby, not revealing, you should cover up tattoos and remove piercings. Hair style should not be a shocker. You do not want to risk any shock (sexy, hip, outré) styles. If in your personal life you pride yourself on being at the leading edge of fashion, just suck it up for a day and back off of that.
Have a writing instrument available, even if you have to ask the secretary for one before you enter.
It may be worth doing a little scouting ahead, or asking a receptionist or secretary what the office norm is, some positions (i.e. in a law office? Sales?) do have higher levels of formality.
Ideally all professionals would be appropriately and modestly dressed at all times.However, I can understand at an event with tables where you are trying to get people over to your table rather than the table across the way that sex sells.
So the company may have different standards depending on if you are giving a class vs when you are needing to be as attractive as possible in a more sales type of role. Even if you aren't directly selling something, when you are in that more of a marketing role that can be a different standard than other events.
It would be nice if you didn't have to use your assets as a woman to make a sales/market the company, etc. I'm sure with guys it doesn't matter if the cut of your suit is a little more roomy vs sexy slim or your hair is a little more mussed up vs rigidly styled. For women it generally does matter. You are likely to be punished for dressing way too modestly (with lower sales and thus lower compensation even if the company doesn't directly act against you) than you are for dressing a bit too sexy.
And every company is going to have a different line on where the too sexy is and how much they are going to say about what you are wearing. After all women have sued for being fired for being a constant distraction.
That said. Dress for the job you want, not the job you have is always a good general rule to follow. You wanna be the girl at the desk at the job fair...or you wanna aim for something higher up the food chain...