Did you read the quote from the WIki that I literally pasted into the conversation for you? I'll repeat it, yet again:

"Only tag construction sites (particularly roads and railroads) if they are planned to be closed for at least six to nine months"

Source is the OSM wiki: highway=construction

"OSM data is often used offline (and therefore may be several months old), only tag construction sites (particularly roads and railroads) if they are planned to be closed for at least six to nine months"

The offline bit is key -- there are many systems that take a monthly dump (or even less frequently) of OSM raw database info and use that to build their own mapping apps.