While <R11> states that non-threaded spacers are allowed for up to 2.5", I want to clarify that is it legal to use a non-threaded spacer that differs in the ID of the VEX Spacers sold?
The full text of R11 reads as follows, with some portions bolded for emphasis:
<R11> Certain non-VEX fasteners are allowed. Robots may use the following commercially available hardware:
a. #4, #6, #8, M3, M3.5, or M4 screws up to 2.5” (63.5mm) long made ofsteel or stainless steel (i.e. similar to those found in the VEX V5 product line).
b. Shoulder screws cannot have a shoulder length over 0.20” and a diameter over 0.176”
c. Any commercially available nut, washer, standoff, and / or non-threaded spacer up to 2.5” (63.5mm) long which fits these screws.
R11-c permits hardware that "fits these screws", i.e. "#4, #6, #8, M3, M3.5, or M4" sizes. Since the spacers sold by VEX have a #8 inner diameter, then yes, a commercially-available spacer with a #4 ID would still be considered legal under R11.
For example, could I use a nylon spacer that matches the specs of what is sold on vexrobotics.com, but the only difference is the ID to accommodate a HS Shaft (0.250" - 0.257")
Not under R11, no, since HS shafts or 1/4" screws are not one of the dimensions listed in R11.
Theoretically, there is probably a commercially available spacer with a legal inner diameter ID, that a Team could claim that they have purchased, bored out with a drill press, and used as their 2.5" long HS shaft spacer. Realistically, going to this length to avoid using the HS shaft spacers sold by VEX is similar to the points made regarding "hollow screws" in this Q&A.