We remember reading somewhere that there is a demonstrable pattern in English: two separate words become a hyphenated compound, then eventually the hyphen disappears. In other words, most if not all hyphenated versions are ‘on their way out’, the only question being how long it will take.
In the case of plugin vs plug-in, the hyphenated version still seems to be the clear winner in the hybrid car market, for example – see Wikipedia page here. Google Ngram Viewer also shows the hyphenated version in the lead, although the rapid decline of the hyphen is obvious from the chart here. See also the pertinent discussion at English Language & Usage Stack Exchange here.
Conclusion: we will write plugin henceforth, except in cases where we want to differentiate something from a (code) plugin, e.g. “they were looking for a plug-in charging point”, which makes it clear that they were not looking for a piece of software 🙂