It is still possible to use aircraft for IR training that do not comply with the PBN requirements?

Interesting question, we've been getting a few questions on PBN recently and have an article in the pipeline. In the meantime, I'll see what our EASA experts can tell us.

Yes it's possible, you can do a PC with only IR helicopter not equipeed PBN. The pilot revalid the rating only IR without PBN part and his privileges will be valid only for IR flight e not RNAV procedures.
Italian Authority ENAC with Information Notice (NI-2020-015) par.4 (italian language):
....Qualora non sia possibile effettuare l'avvicinamento RNP APCH né in volo né al simulatore, i privilegi PBN del pilota non comprenderanno la parte RNP APCH. In questo caso il pilota rinnoverà regolarmente l’abilitazione IR e potrà continuare ad esercitarne i privilegi ma non potrà effettuare avvicinamenti RNP APCH fino a quando non abbia superato il relativo check in volo o al simulatore.

Regards
Domenico

If it is not possible to make the RNP APCH approach either in flight or in the simulator, the pilot's PBN privileges will not include the RNP APCH part. In this case, the pilot will regularly renew the IR qualification and will be able to continue to exercise the privileges but will not be able to make RNP APCH approaches until he has passed the relevant flight or simulator check.