It is allowed, I have it installed myself. Be warned that updating ReShade is a bit clunky and it prompts for updates frequently, it is also not a "one filter fits all" program. If you watch Obsidian Ant's video he talks about needing to switch between different filters in different situations to get the best visual experience. I personally do not use it and keep forgetting to uninstall it.
The problem with reshade for Elite is that in majority of other games it had 2 lightning situations to consider: daylight and night time.
In Elite this is multiplied by the amount of star types, as each one generates a different type and colour of lightning.
Most of the presets I've tried only cater for one lighting type, and therefore look quite bad in other lighting conditions.
I'm using my own preset, which I've spent tens of hours of testing and it attempts to find a happy mid point between all various lighting types.
It's not very pronounced, as I don't like exaggerated saturation and bloom, but it does visibly change the graphics to be a bit more deep, sharper and better deal with shadows.
It's not perfect, as I still get some banding especially when close to a star corona - but it's a small downside of having an enhanced graphics. Drop me a PM and I'll send it to you if you want to check it out.