In the 80's, the FCC put limits on digital signals below 30MHz based on "baud rate". A baud is a raw bit in a digital data stream, it can either be data or part of the channel protocol. For example, a typical serial port like you have on a PC or an Arduino or something might operate at 9600 baud, each "chunk" consisting of a start bit, 8 data bits, and a stop bit. That is a total of 10 bauds, two of them, the start bit and the stop bit, are part of telling the circuit where the data starts and stops. So 9600 baud sends 960, 8 bit bytes per second over the line or only 7,680 bits per second. With me so far?
Okay, so the reason baud rates were used is because digital modes were modulated using a technique calls "frequency shift keying" or FSK. Frequency shift keying would send one tone for a zero bit, and one tone for a one bit. Those tones were detected with a circuit called a tone detector circuit and typically they needed a few tens of cycles of the tone to reliably detect the tone. A higher frequency tone meant you could detect it sooner (shorter time for the detector to latch on to the frequency) and that would give you a higher baud rate. But if you're modulating a higher frequency tone on to an RF carrier, it creates a wider impact on the spectrum and everything else was predicated on 2.5kHz max width voice channels. So allowing a faster baudrate, using FSK modulation, would result in digital modes taking up way more spectrum and thus limit the number of users.
But between then and now, there has been a freakin' Cambrian explosion of modulation techniques because digital signal processing is just math. We have a whole stable of techniques in the barn because of this, And as a result, you can put a lot more bits on a channel without pushing the spectrum bandwidth out.
A lot of people have pointed out to the FCC that making the limit baud rate based was silly if they really wanted it to be a spectrum bandwidth limit. Just make it that, and the experimental folks will compete to see how many bauds they can fit into that space.
I will admit I am biased, I'm one of those folks who got back into Amateur Radio because I was playing around with SDRs and wanted to start trying new modulation techniques. I am not motivated by "QSOs in every state" or every country, I'm motivated by "I just pulled an image off a weather balloon over the Atlantic ocean on 20 meters!" and "I can see my beacon 500 miles away on the KiwiSDR network!" things like that. So this change is really going to open up a lot of space for experimentation for me and I can't wait.