Shatter Burst (Slot 2 Battle Mod) is seemingly the ONLY thing that contributes to you plexus Kill stats.

[PoistyNZ]

Did quite a lot of testing on this. 

 

Shatter burst is the only battle avionic that does so.

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A list of other things that don't:

Liches killing boarders,

Crew killing boarders,

Tactical menu abilities killing boarders,

Tactical menu abilities killing POI enemies,

'Specter' crew killing on POI,

All the other battle avionics,

Ramming ships,

Shooting other ships with railjack weapons

Archwing killing stuff (weapons or abilities),

Your warframe/weapons killing things,

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Relevant Reddit post  

https://old.reddit.com/r/Warframe/comments/mnzkap/how_exactly_do_you_get_a_kill_with_your_plexus/

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Its a bit unusual...