"For the fifth year in a row, I've asked Société de Transport de Montréal, Montreal's transit agency, for the foot traffic data of Montreal's subway."

The issue is that you need to ask for a snapshot. As this is non-sensitive public sector data, it should be available real-time to taxpayers, in my opinion.

Maybe not exactly what you are looking for but BestTime provides relative foot traffic data (also for Montreal's subway station:

Here a free demo page of some subway stations in Montreal:

https://besttime.app/demo/radar?q=metro+stations+in++Montrea...

Thanks but that was a more general comment about public sector data.