You can create a new baseline in the criteria filters by release date:
History of patches and dates versions:
ESXi 6.5 Patch 02 2017-12-19 7388607
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So basically I would just exclude patches newer than 2018-03-20 (release date for 6.5U1g).
Does anyone know if VMware have a list of patches for each "update" release, so it's easier to do this in the future? Would love to have an option in baselines going only for certain Update levels (required by some vendors when running certified solutions).
No, you will have to create it yourself.
Creating the baseline with the lower date the VUM will only download from the selected date back. I recently had such a demand.