The T6.1 suspension rule is the original 'Anti GoKart Rule', written many years ago after some teams brought what were effectively gokarts to the competition.
The rules makers decreed that all cars needed suspension.
So, "what should 'suspension' be"? was asked.
The answer? "Oh I guess a minimum of one inch movement in either direction would suffice", was the answer.
So, the rule has always been vague and it certainly does not preclude beam axles.
There is no rule about wheel rates etc, so suspension on FSAE cars is often 'virtual', assessed by the tech inspectors by inspection rather than measurement.

However, remember, excitation at the contact point is the enemy of grip.. and stiff suspension excites the contact patch!

Pat Clarke