The Schism War is described mostly in broad and general terms so the books have provided me with few answers so far.
Quite a few covenants were destroyed during the wars and while I'm certain a lot of them were burned to the ground with magi and coven folk inside I can't stop wondering what happened to all the libraries. How many magical tomes were carried off. Did magi keep quiet about this as not to be accused of studying Diedne magic?
The problem with the devices that might have been looted, is that they might have been created with infernal Vis (after all why else would they indulge in such depraved practices as whatever they were actually doing) .
Therefore anyone openly using them could be at risk of politically motivated charges of Infernalism. I suspect therefore that those who did loot the covenants pretended that instead the goodies that they acquired were destroyed and so there would be no such risk. After few generations these devices (and even books) could have been left as legacies and therefore few people would know that they were using Diedne loot.
The entrances to the regio surrounding Branugurix was closed and rumors say that some of its leaders escaped into it. But there are still several Diedne covenants that was destroyed during the Schism Wars. I doubt that the fleeing magi managed to bring all their books and enchanted items with them.
There might not be that many Diedne secrets left behind, but there should still be quite a few books and other stuff left.
Maybe the Diedne didn't write any books, same as the Druids (by reputation.)
In an OoH whose economy is based on trade in texts, such Diedne might be resented, willing to consume but adding nothing to the Order. And they leave behind no libraries of special interest.
Or at least didn't commit anything particularly secret to writing. I would expect other Mystery Cults to likewise keep their initiation scripts in oral form only, for the same reason of secrecy.
What sorts of distinctive magic items are you looking for? I would expect the Diedne to have the usual magical bric-a-brac that other Covenants have lying around but their signature ability doesn't translate into enchanted devices. If the Infernalism rumors are true there could be some ugly Hell-devices but as mentioned above nobody will want these, or at least admit to wanting them.
I believe, it is left on purpose vague in the all books, to left room for every Saga. In game, there is no written traces of what/who were exactly Diedne magi, except probably in some very secret archives or diary (House Guernicus, Tremere, the usual suspects).
You can decide that they were really powerful devil worshippers, trying to enslave the Order, then Mythic Europe. Artifacts will be according to this paradigm: books with demon's secret name, rituals of binding, initiation script to various Virtues (or Shadow virtue), imprisoned imps, tainted virtus and various evil sacrifice.
On the other hand, you can made Diedne just another tradition, but because of its sheer number, good organisation and possibly few unique rituals was considered a threat by a some Houses which decide to take them down, to loot their knowledge and insure their place in the Order. Maybe this good collaboration was possible because they had a way to minimise the effect of the Gift ? Maybe some inititiation which could turn any Gift into Gentle gift ? Maybe the ability to teach to several students at the same time and have them work together despite the gift. The Diedne loot would not look much different than any other House's loot. Only certain references in a text or certain runes could hint their origin.
You can decide that they were practitioneer all gifted with Chtonic Magic, without being demonist. Their power was coming from some forgotten deities (either from the Magical realm or a Faerie impersonator), but their real power was their ability to constantly tap into their Chtonic magic, making them much more powerful than a magus of the same age. Here again, loot would not be that different from a regular covenant, but will be tainted by the Chtonic ritual.
For additional ideas, look into Ancient Magic and borrow some unique abilities coming from one of this lost tradition and reskin it to fit your vision of Diedne. Maybe the spoke Aramaic - except that they call it Enochean, giving them access to the power of True Names and such. Maybe they had unique connexion with the Dead (Canaaite necromancy), which would explain how they could be seen as evil worshipper).