Immutability is easy to achieve. Anything distributed widely enough is effectively immutable. Anything published on the cover of Time magazine is immutable. Anything commited to the linux kernel is immutable. Anything distributed on thepiratebay is immutable. Anything put in the Ubuntu package repository is immutable.

You can amend it and publish new versions but you can be sure that somebody somewhere will have a copy. You want to make your paper trail immutable? Sign it and publish it widely using any medium you want. Burn it on CDs and mail them to random addresses. Tie them to balloons and release them.