Do attack speed debuffs affect champions with auto-attack resets and auto-attack cancel spells?

Let me try to answer your question by taking another example: as a Wukong in lane, using my E will allow me to dash on the enemy champion and give me a buff in attack speed(AS). My burst is enormous since I would follow up with an auto attack(AA) and then reset my AA timer with my Q for even more damage. Then I would take profit of my AS buff and throw 2 more AAs before decoying away.

Now if my enemy is a Malphite. As soon as I will jump on him with E, Malphite with then use his own E, which will reduce my AS.

My burst will be reduced substantially. Why?

1° After I use my E, I will take more time than usual to do my first AA and my Q. This will allow the enemy to either leave or attack and trade.

2° I will never be able to launch those 2 AA's before decoying out because it will take too much time for me and I will be taking too much minions aggro.

3° Sometimes I will unintentionally cancel my AA just because I'm not used to having my AS reduced to that speed, which is still in advantage of the enemy malphite. (or any champions that reduces my AS)

The burst has been reduced indirectly because of the AS reduction, even though the potential damage has stayed unchanged.

The answer is yes, att speed debuffs can reduce potential burst damage.

Multiple people discussed att speed debuff over the forums of League of Legends: League of Legends NA Forums

As mentioned there:

Well the reason for this is attack speed slows slow your animation

Even if you play a champion which has an AA reset timer. The actual animation of your AA gets slowed down as well.

So in your example of an Riven / Nasus setup, the wither from Nasus will slow down riven's attack animation, thus reducing her burst.