If you use the “Reverb” effect, you can keyframe the effect.
If you use the “ExpressFX Reverb” effect, you cannot.
To keyframe the effect, click on the FX button on the track, add that effect, and the click on the “Configure FX Automation…” on the far right top. Select the items you wish to keyframe and then you can control them with envelopes on the timeline.
Edward Troxel