Technically, tags can be created at any time. When starting a new question in G2G, you would just type in the name of the tag and if it doesn't already exist, it will be "created". (It is bit more technical than this, but makes the point).
For instance, if you were working on a new surname with a uncommon spelling, there is nothing stopping you from using that tag.
For the existing Alsace tag, see: /g2g/tag/Alsace and /genealogy/Alsace.