Rear entry dropouts on a geared TT bike usually mean the chainstays are so f-in short that you couldn't get the wheel in with a fully inflated tyre using normal dropouts. They are not for optional fixed use, since riding fixed in TTs was already a niche for weirdos by the time lo pros arrived. If anything, there are probably more people TTing on fixed now than there were in the 1980s.
Rear entry dropouts on a geared TT bike usually mean the chainstays are so f-in short that you couldn't get the wheel in with a fully inflated tyre using normal dropouts. They are not for optional fixed use, since riding fixed in TTs was already a niche for weirdos by the time lo pros arrived. If anything, there are probably more people TTing on fixed now than there were in the 1980s.