Enforced poorly, but enforced.
Isn't any enforcement going to sting, if you are at all devout?
There's not a lot to support the notion that it stung sufficiently to motivate reasonable people to take up arms if they were complacent enough not to do anything about it until the Stormcloaks stir the veritable hornet's nest to create a substantially different environment to wit:
So we have Alvor, and it seems pretty clear from what he says that the Thalmor have been arresting people for Talos worship for quite some time (decades maybe?) though not until after Ulfric's release from prison and the rise of the Sons of Skyrim.
That's not what I take from Alvor's dialogues. It's highly improbable that the Justiciars haven't been detaining people for Talos worship since at least the Markarth Incident, but the point of Alvor's dialogue isn't about whether they arrested
anyone over the course of time, it's that the Stormcloak rebellion is the catalyst for the extremely heightened level of enforcement. The Sons of Skyrim is another term that's used for the Stormcloaks and Alvor is stating that people were inconspicuously worshiping Talos up until the Stormcloak rebellion largely without incident.
My perception of Ulfric is that he orchestrated the civil war with a fair amount of planning and manipulation. He chooses to kill Torygg not only so he can ultimately claim the throne of the High King for himself but as a symbolic gesture of his declaration of war against the Empire. He knows that the Justiciars are going to react and he knows that their reaction will result in an environment that will push an appreciable number of Nords to his side (including my character in my Stormcloak play-through notwithstanding that she has the same perception of Ulfric).