As the title suggests, Onatel continues to steal mana after death.
This was done several times to ensure credibility, but after several tests I found this:
When Onatel has used her special and her mana steal is in play, killing her with tiles works normally. She dies and her mana steal is removed.
However, in the same scenario, if Onatel has her mana steal in effect and you kill her with Special Skills, her mana steal remains, and the consequential tiles that follow if matched to the colour the steal was in place on, have the appropriate mana stolen as if Onatel was still alive.
The Special Skill clearly states ‘This effect can’t be dispelled, but gets removed if the caster dies’.
As you can see, Evelyn has struck Onatel here and Onatel has mana steal on the three centre heroes.
Lianna uses her ‘Perfect Shot’ resulting in Onatel falling, yet the mana steal stays in place. The following yellow tiles then give 0 mana (Mana Steal was at 100% in this case).