Dax makes many references to having lived in the TOS era; the affinity with the old school tricoders, the uniform, being felt up by McCoy, etc.
So we know that a previous host (Torias) was in Starfleet and another (Emony) has wandered about on Earth playing hide the sausage with the good Doctor. Trills, clearly, aren't that alien in that time.
So why, as Jadzia is preparing to board the Enterprise, are her Trill spots being masked? If Trills were happily wandering around the galaxy in the TOS era and before, why was it necessary to de-Trill her?