Indeed. And that's why you should vote with your wallet. Support those who DO inform this. At least Asus lists the range on their MG279Q model (check the last section, "Note"):
http://www.asus.com/Monitors/MG279Q/specifications/
If people would only buy those models that are openly advertising the range, the competitors would slowly have to follow suit, as they couldn't afford not to. Retailers can also come into play and specifically keep requesting for this info, making manufacturers more aware of customers' desire for this piece of information.
Alternatively, the customer will have to search for in-depth reviews that take note of this range. At least
http://www.tftcentral.co.uk seems to include this spec in its reviews.
One last thing to consider:
In general terms, manufacturers tend to advertise only the best feats of the monitor. So, when some specific piece of information is "hidden" or outright not available, there's a high probability that it's lacking in quality, as well. In Freesync's case, the range might be too short/limited.
Then again, input lag is a clear exception for this. I don't think ANY manufacturer advertises this, not even in models that are spectacular in this regard. It's like the manufacturers have a common agreement (cartel) for not listing it. Very frustrating.
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