MySQL 6 was used some 10+ years ago and has seen some alphas. Over time most things in there have been backported into the 5 series and some ideas for 6.0 didn't work out (i.e. Falcon storage engine) and were stopped. When/how 7 was used I'm not sure, but jumping from 5.7 to 8 simply drops the first digit, which was meaningless for a while already. (jumps from 5.5 to 5.6 and 5.6 to 5.7 were huge as well)