Impact of the channel length on the threshold voltage in CMOS transistor?
The short answer is - yes.

The slightly longer answer, it depends on the fabrication details of the transistor and it's targeted use case.

In all cases, below a critical length \$V_{TH}\$ decreases with decreasing length.  However once this critical length is met the \$V_{TH}\$ can increase, stay the same or decrease.  Here are some pictures.


image 1: flat


image 2: decreasing


image 3: increasing

There is also a correspondance between Gate Oxide thickness, the S/D structure design  and this critical length.