Yes, the feature is available since a long time. Have a look at the Time in Zones tab under the graphs of the Activity page.
PS: Please avoid cross posting to multiple threads.
I understand the question differently. It's not about the distribution among zones in single activities but aggregates (total training time in zones). And I didn't find anything about this so far. Thx though!
OK.I see. Aggregates over multiple activities or over a certain time period are indeed not available. You can try requesting it at Garmin (for example at https://www.garmin.com/en-US/forms/ideas/). Perhaps there isn't any sufficient demand for such feature, so the feedback can help. Here, on the user forum, it is not very likely to be noticed by Garmin.