[Tagging] Multiple tags for one purpose

[Valor Naram]

Hey,

some long time ago I wondered why we have two tags for one purpose
sometimes? For example: A mapper can use either the tag `contact:phone`
or `phone` to add a phone number to the database. I think this makes
the database dirty and for developers - like me - it's annoying to
support two or more tags for one purpose.

We need a system to prevent or instinct the usage of two or more tags
for one purpose. I suggest the following behaviour:
1. Negotiating which key can be considered as official.
1.1. A key which has been approved should be the official key but other
factors like usage or improving can be considered.
2. Deprecating the other key by following the introductions on the
https://wiki.openstreetmap.org/wiki/Deprecated_features page.
2.1. Opening issues for incorporating the official key into presets and
removing presets with the old key:
2.1.1. https://github.com/openstreetmap/iD/issues
2.1.2. https://josm.openstreetmap.de/newticket
2.1.3. https://github.com/westnordost/StreetComplete/issues
2.1.4. https://github.com/osmandapp/Osmand/issues
2.1.5. https://github.com/simonpoole/beautified-JOSM-preset/issues
2.1.6. ...
2.2. See how much the official key is used among mappers on
https://taginfo.openstreetmap.org/ and also see the usage count for the
old key. (For a period of 2-3 month)
2.2.1. If things are developing well and the official key gets more
usage and the old key less usage then we won't need a mechanical edit.
2.2.2. If things aren't working for some reasons like there are not
many real objects that can be tagged with the official key then we will
do a mechanical edit but keeping the `check_date` and
`review_requested` keys intact.

I also had an interesting talk with Quincy Morgan at
https://github.com/openstreetmap/iD/issues/6529#issuecomment-524437983
about "fragmentation" as Morgan calls it.


In my opinion this is a topic we should consider working on and
creating a wikipage to describe the "defragemtation" process in
general.

Cheers,

Sören Reinecke alias Valor Naram
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