I have a non-legal (as in illegally downloaded) XPart EPC that dates back to early 2011 which I have running on an older Windows XP laptop, it's a bit cumbersome as in order to run it, one has to set the clock back to early 2011, but it allows me to check on diagrams and part numbers, pretty much in the same way as one would have done with Rimmer before they decided to remove older, NLA parts. The only disadvantage is that parts that may have had a change of reference/part number since 2011 would obviously not give me the latest reference...but there are not that many changes anyway.
Regarding the recent change with Rimmer no longer referencing NLA parts, my XPart-affiliated garage told me that XPart has been doing the same for their own, current EPC, which runs strictly online now (as opposed to my older copy which runs locally). As Rimmer uses the XPart EPC as the background engine for their own customer front-end, this might also be the explanation. My own garage now has to rely on previous EPC programs to find those part numbers that are no longer referenced and of which he may still have NOS.

Could XPart provide us, MGR clubs, with an EPC that would just show us diagrams and part numbers, including NLA ones? Since there's no obvious business gain for them to do that, since NLA parts are...well, NLA and therefore not orderable from them, I highly doubt they would even consider it.

An online alternative to Rimmer that I know of, to check on diagrams and part numbers, is https://allbrit.de/UGCtree.cfm?SPRACHE=EN, but they don't have the 260s there, and are a lot less user-friendly. The diagrams they use also seem to be considerably less current than the Rimmer ones.