Citing older adjunctive chemo agents as a rescue of downstream I/O 1.0 agents is not a strong argument.
Tamoxifen has been used for decades in ER+ breast cancer, as this cancer can be hormonally driven. Contraindicated in bleeding diathesis, liver disease, and endometrial problems. No evidence or trials exist for efficacy with I/O 1.0 agents.
Herceptin is used in HER2+ BC, which as you know, is only 20% of breast cancer cases. There is no evidence and no planned trials for combination with downstream I/O 1.0 agents.
Xtandi is an anti-androgen drug used in prostate CA. Similar to the effect of Lupron, it would be unfeasible to address cancers other than prostate with this drug. In fact, it could worsen many cancers. No evidence exists for combination with I/O downstream 1.0 agents.
That greatly expands the realm of single agent IOs that have shown efficacy on their own.