How to reproduce:
1. Open the user's attached "Universal RP particles issue" project
2. Open the Scene "SampleScene"
3. Open the Graphics Project Settings (Edit->Project Settings...->Graphics)
4. Set the Scriptable Render Pipeline Settings to "UniversalRenderPipelineAsset"
5. In the Hierarchy select the "Particle System"
6. In the Inspector expand the "Renderer" section of the Particle System Component

Expected results: "Reflection Probes" is not grayed out
Actual results: "Reflection Probes" is grayed out

Reproducible with: 7.5.3 (2019.4.24f1), 10.4.0 (2020.3.3f1), 11.0.0 (2021.1.2f1), 12.0.0 (2021.2.0a12)
Could not test with: 2018.4 because the URP was not yet available

Note:
- To use the Reflection Probes on the Particle System you have to use Built-in RP, change the Rendering Path to "Forward" and change back to URP