I used to encounter the phrase 'whiskey tenor,' but it's old-fashioned now.

I think it meant a throat in bad condition (cracked, hoarse) probably combined with poor control of tuning.   

It is definitely a bad thing in a singer. However, the world of pop music has long tolerated singing styles (like 'rock belted') which damage the voice, so the connection between alcohol and a raspy throat is now weakened.

Sorry I don't know anything about Suzanne Pleshette and Mariella Frostrup.