For about 2 years now, you've needed to be logged into strava (a free account will work) to zoom into strava.com/heatmap
beyond level 15 or so. That still works with my free account as of this morning.
They have a comparison page which shows the features available in free accounts vs the monthly subscription. One of the paid-only features is Personal Heatmap, where they take every run/ride/training you've ever done and generate a special map showing the places that YOU have been. This is different from the Global Heatmap (linked above), which shows EVERYONE'S anonymized travel on one universal map.
I expect the Global Heatmap will stay available to anyone with a free account, and Personal Heatmaps will require a paid subscription.