At the moment there is no agreed way to tag a peninsula, although it's an important geographical feature.
This proposal adds no new objects, just tags. This agrees with what a peninsula actually is, not an object per se, but a coastline shape. That's why peninsulas should be tagged on coastlines, their natural boundaries.
Number in keys
A problem with peninsulas is that they often overlap each other. There can be a number of levels of peninsulas on one patch of coastline. See the following image:
That's why I propose tags that have a key with a number, like peninsula:1 or peninsula:4. So for the example in the image we would have:
Which number should I use if I just want to tag a small peninsula? It should be high enough so that all other peninsulas can get their number. A number that is too high shouldn't be a problem, but a too low number could lead to renaming a whole set of coastlines.
There is one more problem with this tagging, and that is in case a peninsula stretches through several countries. Then the peninsula has several names, and data consumers couldn't figure out if neighboring peninsula:0=Peninsula of Europe and peninsula:0=Poluotok Europa are the same or two distinct peninsulas. That's why we should always use language extensions and have peninsula:0:en=Peninsula of Europe, and that would stretch throughout the whole peninsula.
peninsula:X:wikidata=* A good practice would be to use the wikidata tag because that would unify a peninsula better than other tags. A name of a peninsula can be different even in the same language, if there's different names for it in different parts of a country. Also, a wikipedia tag can differ because they don't have to be in the same language.
Why not relation?
Relations are in a way a better way of mapping a peninsula, with one object per feature rule. Also, it removes the need for somewhat clumsy peninsula:X tagging. But the problem is obviously too much members in a relation for big peninsulas. There are other problems, but this one makes relations simply unusable for this purpose.
Why not node?
Nodes can't show the exact borders of a peninsula.
Comments are welcome in the Discussion part of the page.