I remember when this first came up - I'm not sure the cops ever made a habit of releasing manifestos.They've been not publishing manifestos for a long time.
All it does is give publicity to mass murderers.
Much better for them to die in ignominy and nobody knowing what their 'demands' were.
The Unabomber got his printed as a response to one of his demands, if I recall.
After that... I can't think of one.
The shooters with manifestos almost all released it themselves, I think. Left them as social media posts and so on.
I can't remember the police ever finding one as evidence and then releasing it if the press didn't already have it.
I'm sure it has happened occasionally - there are so many shootings that it just seems it must have.
But it doesn't seem to be common.