|This tag page describes a namespace. Use it as was: lifecycle prefix rather than a simple tag.|
The was: lifecycle prefix can be added to tags that relate to features that don't exist but have an high probability to be re-added by a non surveyed edit, as they were present there before (so can for example still be seen on commonly used imagery or import sources. In most cases, in OSM, if you find a object in the database that don't exist on the ground, whether it existed or not, is an error or not, you should just delete it. But in some rare circumstances, when those features still appear in outdated sources still used to enter data in osm (old imagery, import sources, etc.) you can use this tag to warn other mappers not to re-add those features.
How to tag
Add the namespace "was:" to all keys which are no longer relevant to the current state of an object without changing the geometry (let it a node if it was a node, a way if it was a way). You should treat all the tags on an object as a set of facts about the object, and prefix the keys of those facts which are no longer true as a result of being non existent.
Adding a note=* to explain why you just didn't delete the geometry is highly recommended, else, someone might just delete everything again.