Why can no journalist get this right? Three of the six stations have washrooms. VMC has TTC ones, 407 has GO ones in the bus terminal upstairs, and when I went on opening day I was surprised to find a public washroom in the YRT terminal on the north side of Pioneer Village. Obviously intended for their drivers but open to public, sign and everything. Edit: I never saw this one mentioned online but the YRT "terminals" webpage lists public washrooms "when construction is complete" at PV--while the area is a bit torn up the washroom definitely looked open on sunday. GO's 407 washrooms are listed on GO's site. I think the bigger failure here is the TTC subway map, and station signage (at least at PV, don't recall 407) not showing the washrooms since they're not TTC owned.
Also, York U has washrooms inside the york lanes retail complex about 1 minute's walk from the doors of York University station, open fairly long hours most of the year (a quick search shows they're open at least 7AM-12AM weekdays, and open weekends as well).
So realistically only Finch West and Downsview Park don't have washrooms. It should be noted that immediately south, Sheppard West and Wilson both have washrooms, and Yorkdale has them in the mall maybe a 3-4 minute walk from the platform.
Downsview Park is the only one that I'll say is a bit of an issue because of the GO train service...if you get there and miss an hourly train, there's nothing nearby, you'd have to pay an extra fare to go back to Sheppard West and return (thankfully only one fare as I believe those washrooms are in the fare paid area). I guess long term with 15-minute RER it's not as big of an issue, but for hourly it's not great to lack washrooms. At least when the trains eventually come they have washrooms.