Hmm.. I'd say that anything from https://www.warhammer-community.com/legends/#warhammer-40000 should be allowed. 
For a Causal Narrative Format like Crusade it would be okay and allows people to play with older models/ convert and kitbash one up.

But would say that the model should be WYSIWYG (What you see is what you get) as much as possible, is a fair requirement.

"EDIT" - Oh, but just realised, if there's updated version in any of the new publications. we use the newer one (I.E Trueborn in Drukhari Legends) 

-Vern

For weapon systems that predate the Index, like Imperial Guard Hand Flamers, Griffon Mortar Carrier and Assault Cannons, I assume we just treat them as closest available option (Plasma Pistol, Wyvern Mortar and Autocannon)?