Visualize a polyline with decreasing opacity towards its ends in QGIS

Using the geometry generator I was able to create the following:

The idea is to overlay transparent parts of the line over each other, with each substring a bit shorter than the one create before.

This is done by several layers of line_substring($geometry,$length-$length*0.1,$length). line_substring creates a part of the handed $geometry based on starting and end points along the line. This exemplary line only goes from 90 % of line length until the end. The next layer could start at 80% of line length, etc.

Depending on your background you might need to do a finer graduation that 10 % of line length in order to create a smooth appearance.

Another possibility is to apply menu processing / toolbox / explode lines and expode the line, i.e. to create a separate line from each line-segment. Now you can color each segment of the original line with a color ramp from transparent to color to transparent, using the id of the exploded lines (i.e. the segment-no. of the original line).

To do that, on the exploded line, change the color to a data driven styly, using the assistant:

In the assistant, set $id (meaning the id of the current feature) as the source, load min/max values (the number of lines/segments). You can change this values manually to adapt the style.

Than apply a color ramp and set the start and end marker to 100% transparency, clicking on the color ramp and changing the markers (you can delete an add markers and change their color):

This gives you the style of the blue line. The yellow line shows the original line with an offset to show the original shape of the line:

If you look close, where the lines/segments touch, they overlap and thus on the transparent parts, you have some darker dots. To remove these, simply change the end cap style from square to flat: