This stuff was used at some point to fill gaps around a threshold in our house. Could it be asbestos?
Almost anything "could be asbestos" and without lab testing you don't know.
That eyeballs to be Homosote®, a soft cellulose sheet product (can be used as an inexpensive bulletin board - tacks go in easily) as opposed to the more common hard asbestos-cement boards, but it "could" be some softer grade of asbestos for insulation purposes. Or it may be harder or softer than it looks in a picture.
What's the history of the house?