The iconic font library recommended in the documentation is no longer being maintained, and should not be recommended as one of the two recommended icon solutions for bootstrap. The ssl cert on the documentation site at useiconic.com has expired several times and is presently (July 18 2019) expired. It appears that the firm who made iconic was acquired and the buyer is uninterested in maintaining it.

see

iconic/open-iconic#45

https://waybury.com/