For the pairings being illegal.
Faction Pack v1.2, the current official PDF, legal for matched play from 26 August 2026, prints UNIQUE: ENGINES as a banner above the Contagion Engines detachment page, and restates it inside the Warped and Rusted Animus rule text: this detachment has the ENGINES tag and cannot be taken with another ENGINES detachment. It prints UNIQUE: FLYBLOWN above the Flyblown Host page and restates it inside Verminous Haze in the same way. The Wahapedia mirror, which reflects Faction Pack v1.1 and Munitorum Field Manual v1.2, records which other detachments carried each tag: ENGINES on Mortarion's Hammer, FLYBLOWN on Champions of Contagion. The core rule is that no two detachments in your army may share a Unique Tag, so under that reading both pairings are illegal.
For the pairings being legal.
The Munitorum Field Manual v1.3, read on 27 August 2026, is where Unique Tags are published per detachment. It lists no Unique Tag on any of the nine Death Guard detachments, and it goes further: it prints an explicit UNIQUE TAG REMOVED note on exactly four of them, which are Champions of Contagion, Contagion Engines, Flyblown Host and Mortarion's Hammer. Those are precisely the four the older mirror records as tagged.
Three observations make that look deliberate rather than accidental. Death Guard is the only faction with that note: we checked ten other faction pages on the same day and found zero occurrences. The badge slot still works: the Drukhari page in the same release still prints UNIQUE: WYCH CULT, UNIQUE: KABAL and UNIQUE: COVENS in the same position. And the four match exactly, which is what a targeted change looks like and not what a rendering failure looks like.
So our reading, for what it is worth, is that the tags were removed in this update and the Faction Pack's detachment pages have not caught up. We are not confident enough in that to build your list on it.