In true random fashion, I happened to notice that the Manaclipper still has 'no entry' signs (I didn't remember it had any).
Made me want to check the barge going through the Phanauet Channel then, and that thing has one too.
In both cases the text is only there while docked, however, with empty signs while sailing (guessing they were always like that).
So I suppose whatever the reason might be for the disappearance, it's not the language or having text there in general.
(Of course, they could have just forgotten to blank out these ones...)
Adoooulin has some similar signs, but I don't remember if they ever had any text. They kind of look like they might have some faint, illegible text (made so on purpose), so I doubt those were altered in a similar fashion.
Curious case of curious is curious.