Just chiming in as a network tech that deals with these terms regularly.

802.11, sometimes called 802.11 prime, to differentiate the first protocol from the 802.11 (WiFi) group. This protocol was not really every in widespread use. A few early niche cases, but it was quickly supplanted by 802.11a and 802.11b.

The b standard was one of the first major WiFi versions to see adoption, which used DSSS, or direct sequence spread spectrum. Which fell by the wayside because OFDM was faster and more efficient, which led wifi speed increases from 802.11g, through wifi 4 (802.11n), WiFi 5 (802.11ac), and WiFi 6 (802.11ax). The more recent versions use QAM (wireless N+), which augments OFDM with amplitude modulation.

Beyond QAM, speed improvements at this point are minimal and usually require wider channel widths to get any significant improvement, so 802.11 has focused on multiple access improvements and since 802.11ac, have been making improvements to MIMO. They started with SU-MIMO, then one-way MU-MIMO, then two way MU-MIMO.

I haven't read up on the changes in WiFi 7 yet beyond 6Ghz being added. I'll look into it after it's been fully ratified.

Long story short, they moved to 5Ghz and eventually 6Ghz, because there isn't enough channel width in 2.4 for WiFi 5, and 5ghz was getting a bit difficult to sustain for the speed they're trying to hit, so 6Ghz is the next logical step.