Short answer: yes, psilocybin can still have subjective effects while taking diazepam, but they are often muted. Many people report less intensity, less emotional release, and fewer insights when a benzodiazepine is present.
Because diazepam can dampen the experience and is sometimes used to reduce it, some choose to taper beforehand to allow a fuller session. Do not change or taper medication without guidance from your prescriber.
If tapering is not possible, set expectations accordingly and focus on set, setting, and integration practices. I cannot verify any specific biological effects independent of the experience.
For more context on this decision, see this Q&A on tapering diazepam before psilocybin: link.
Stay safe.