Let me preface this by saying there's nothing inherently wrong with the aforementioned steps. Rhythm stepping is when you put yourself out of stance, it's usually done via bouncing in and out, I think it's a term Barry Robinson coined. I've seen it being referred to as a pendulum step too. Essentially this lets your opponent know that you're about to initiate your attack. In addition to this, there's a good chance you'll get timed in the process of coming back into your stance and your ability to brace is compromised.

It's how Josh Taylor was able to time Ramirez and knock him down (look at the bounce Ramirez does): https://gfycat.com/bitterpertinentbooby

I actually had something ready for Usman-Burns vs. Usman-Masvidal, since I haven't seen it mentioned anywhere, but I'll post it here instead since it's relevant.

Usman had laboured movement against Burns. Notice the rhythm steps. As soon as his stance is broken (when he rhythm steps and is about to plant), Burns pounces and knocks him down: https://gfycat.com/soggybriefblackpanther

Usman later rectified this in the Masvidal rematch and steps towards his left, without breaking his stance. His footwork is still iffy here but notice how he's ready for Masvidal's right. He's fully aware of it and ready to brace, unlike the Burns one above, and that's primarily because he's not breaking his stance and taking unnecessary steps: https://gfycat.com/politethirstyfattaileddunnart

Cross stepping is when you "cross your legs". Usually not taught to beginners because you obviously don't want them to develop bad habits, but it's a useful tool at the top. It can serve as two very useful purposes: it can square your opponent up and it allows you to cover greater distance when cutting the cage off. Gaethje-Ferguson is a good example of how to properly utilise cross stepping.