This was implemented in T90973: T9. When deleting board or topic, contributions should move from Contributions to DeletedContributions but seems to have broken since then.
E.g. I Deleted these 3 topics (one, two, three), but nothing at all is showing up in the user's Contributions or their DeletedContributions.
Amended description from 2018-12-14 (UTC): When an administrator deletes Flow content, said contributions are attributed to the deleting sysop and logged in the sysop's personal DeletedContributions log. This is unlogical, causes copyright issues on edits being misatributed and most importantly, T118024#2314965 (real example: I've just deleted threats towards an editor and now those edits appear on my personal deleted contributions log, which as you can imagine, makes no sense and is unnaceptable).