Was under the assumption from multiple people that the past raid and dungeons are farmable for exotics even though they are a low drop rate with no boosts. Me and others with over 20 plus clears this week on the final boss with no drops between all of us. Are they not farmable and only three characters per week again? Bungie should really give some sort of pity with people that have a considerable amount of clears with no exotic drops. RNG is horrible.
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At the moment there is no confirmation from Bungie to whether they're farmable or not. Outside that, don't think anyone has posted any community sourced results (with evidence) about it. As it stands the featured system is gone and the legendaries are farmable, but until shown otherwise the exotics should be assumed as going back to 3 chances a week.