A couple of people on Twitter seem to think the new mulligan rule will be in effect at WMC. Is it true or am I mistaken? I haven't found any official confirmations for this.

It is several months ahead of us.
We will know in due time I guess. Announcement will come in due time

The mulligan test performed in Vancouver is not allowed in sanctioned matches at this time. Whether or not it is again added for the WMC, or even changed in policy for all events before the WMC, is unknown. (And, to be fair, we really don't need to know - only the WMC participants have a stake in that, and they'll find out before it matters.)

d:^D