It's important to define what "legal" means.
14 CFR 65.47 is FAA regulation that has to go through a whole comment period and stuff if they want to change it. The wording only applies to control tower operators but I guess it's interpreted to mean 2152s in general? I'm not clear on that point. Anyway, this says that you can't work more than 10-hour shifts nor can you work more than 10 hours in any 24-hour period,
unless you have an 8-hour break. No rule on minimum shift length.
Then the 7210.3 2-6-7 is an internal FAA document and they can change it with the stroke of a pen... unless NATCA pushes back and says it's a change in working conditions, which is what happened. This is the 9-and-8-hour rule for time off between shifts which they're changing to a 10-and-12-hour rule. Again, nothing about minimum shift length.
Finally the Slate Book Article 34 defines the basic workweek as 5x8 unless the employee voluntarily agrees to an Alternative Work Schedule. I guess this is what
ManBearPig means about "can't do a 6hr shift" because a Flexible Work Schedule is just normal 8-hour shifts but with flex time, and the two types of Compressed Work Schedules are (1) 4-10 and (2) 5/4-9.
So if you wanted to write a schedule with 9-9-8-6-8 or whatever you would have to have NATCA and the FAA negotiate an MOU overriding Article 34. That's certainly doable, and NATCA should explore that if the workforce wants that kind of schedule. You just have to be aware of what it would take to make that legal.