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It was created, but never implemented into the game

With the jingle update for other combat skills coming up maybe this could be looked into?

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about 2 months ago - /u/Jagex_Stu - Direct link

Necromancy and Archaeology do indeed have a unique jingle for level 99 and 120. (Other skills reuse their second "over 50" jingle.)

Had a dig into the code and looks like they are used at the specific levels of 99 and 120, not the range between 99 and 120 (wherein they use the "over 50" jingle), which is possibly why you might think they aren't implemented?