Interesting. I was just reading over Jeff Tallman's 2014 presentation "Troubleshooting and Tuning ASE Statement Cache Query Performance", presented at the ISUG-TECH conference a couple weeks ago, and saw this at the bottom of one of the pages:
Sort Merge Joins = bad bad bad (unless 2 #temp tables)
Likely means an index missing or similar
So, it may be that no matter which version of ASE, the Merge Join is something to avoid if possible.