Analysis of information sources in references of the Wikipedia article "English plurals" in English language version.
But note that 'Unixen' and 'Twenexen' are never used. It has been suggested that this is because '-ix' and '-ex' are sometimes Latin singular endings that attract a Latinate plural.
4a plural counsel (1) : a lawyer ...
[O]f the 136 distinguished consultants on usage polled for the 1975 Harper Dictionary of Contemporary Usage, 49 per cent responded that they use 'The data is...' in writing. And, in casual speech, 65 per cent use data as singular. Those who defend 'The data is...' often point to the fact that agenda is also, strictly, a plural, but is nearly always regarded as a single list and takes a singular verb. You'll probably never hear anyone ask: 'Are the agenda interesting?'