For everyone who don't have the Sturm catalyst sorry to say it but if you didn't get it before 2025 I don't think you can.
I have now been playing on nessus in +25 hours I have over 30k kills I tried switching between titan warlock hunter, tried restarting game, tried solar void arc stand stasis prismatic, tried in a group and solo tried heroic events public event and lost sectors, I tried triggering it with completing catalyst for other weapons. Still nothing. So I don't think it's possible. Don't be like me don't waste your time. wait on bungie to fix it. If I somehow get it I will let you all know.
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I can't say this with absolute certainty, but I'm pretty sure I remember something being said about catalysts for older Exotics having a lower chance of dropping than the newer ones. It seems odd I know, but I guess Bungie figured players would be more anxious about getting the newer ones over the older.