The tag functions doesn't seem very useful. The tag wiki only states the definition and the questions just... contain functions. In terms of reviewing, there aren't really any differences between questions with just functions or with all the implemented code. Users may just end up using the tag if their question contains one or more functions. At the worst, it could be used just because the code already "functions" properly.
I don't think synonymization would work, so burnination (or not) may be the only options.