We have gone plural. 
Well, at least for themes and #plugins, to match #theme-components. It will be on a category-by-category basis, so there is scope to tweak a couple more if the mood takes us. 
And they’ve been reverted…
Someone has kindly pointed out that pluralising them would cause a name-clash with the existing #plugins and themes tags. I have rolled back the changes while I reconsider the options.
And another pertinent example for the ‘singular’ camp: