The description of the option "--show-del" on http://www.htslib.org/doc/samtools-consensus.html goes as follows:

--show-del yes/no

    Whether to show deletions as "*" (no) or to omit from the output (yes). Defaults to no. 

However, the actual effect in Samtools 1.18 is reverse: "no" doesn't show asterisks, while "yes" does.