Is there any detrimental effect on signal reception when mounting crossfire antennas in the Immortal "L" orientation compared to the traditional "T"?
The short answer is, yes. According to testing done here there is an observed 3db loss when folding the bottom half of a dipole which would be the same effect as cutting your output power in half.
See this graph from the link above. At the bottom in red is the bend of the antenna and the blue line is the gain.
The 'T' antenna is called a dipole and it's a very common antenna design. Changing the shape of the antenna will almost always change the antenna's gain and radiation pattern. Unless you have tested very carefully or are following a known and tested antenna design, chances are you aren't improving its performance. This article describes what one person went through to test folding a dipole by 90 degrees. Other things to keep in mind, the L shape allows your antenna to be polarized in two directions, while it might have worse reception under certain circumstances it may be better in others. This could be especially useful on a quad in situations where the orientation of the antenna would not be straight up and down.