Heartless One gets integrated into Gluttonous King Beelzebub and can still be used, it was just renamed into a sub-ability of Gluttonous King Beelzebub called "Soul Consume". From the Light Novel itself:
"Merciless, a power I thought could come in pretty handy, was subsumed into the others—a pity, but it was still active within Soul Consume."
Note that Rimuru here isn't talking about the ability being downgraded or anything. It's mentioned earlier (LN vol. 3) that all of Rimuru's fused abilities are "downward compatible" (though some aspects might be removed if Great Sage deems them dangerous, like how Gluttony doesn't have the "make subordinates into hungering Zombies" trait). What he calls a "pity" here is just his "collector's spirit" lamenting that a cool-sounding ability is no more (in name at least) and his "Skill count" has been reduced and streamlined, I guess. It's like how Great Sage "sacrificed" Shizu's Degenerate Unique Skill, but it's effects are all neatly integrated into Wisdom King Raphael and still usable. It actually appears like Great Sage used countless different combinations before going with "sacrificing" Degenerate to evolve herself probably because she knew that it was like a memento of Shizu to Rimuru on a sentimental level.