what's randuin's going to do now?

Actually, it shouldn't. True damage can't be reduced.

See it that way: Randuins reduces crit damage by 20%. Infinity Edge makes 20% of your crit damage into true damage.

But that means that 80% normal crit damage still exist. This will most likely be reduced, so basically Randuins is reducing the damage just by 16% instead of 20% now (0,2 * 0,8 = 0,16).

it will reduce the crit damage so say an Crit of 100 to 80 damage

20% of the 80 will be converted so 16 true damage for example

They haven't changed it yet but they definitely talked about it