If it's not in the Codex, he's supposed to disappear an be used moving forward as a standard missionary.
Really? That's obviously the intent
, but to use a different example, Daedalosus was obviously replaced by the Technoarcheologist in the new Adeptus Mechanicus Codex, and as far as I'm aware there's technically nothing stopping you from still using him in a 9e Adeptus Mechanicus army; his datasheet is still floating around, it still has the ADEPTUS MECHANICUS faction, and it's still valid for 9e.
I could be wrong, of course, and I haven't read the new codex, but I don't see why the same wouldn't apply to Taddeus.
Hell, as far as I'm aware, there's nothing stopping you from taking a Tech-Priest Enginseer from the Astra Militarum codex in a Adeptus Mechanicus army; he'll lose a pip of BS
, have a worse pistol, and go to a 6+ invulnerable save with no DOCTRINA ASSEMBLER keyword, but he costs 20 points less, he's Elite instead of HQ
, and he can repair Knights.
EDIT: Also, this is the wrong forum for rules questions.