Removing Yellow Age Stains on Vintage Crochet Afghan?

I would suggest washing it in a large lingerie bag for sure so as to not twist it as the threads may be weakened from age.
Use a gentle or delicate cycle and use plenty of water (full load) so the agitation will not damage the fibers. You can add other light colored items but hot too many - slow gentle swish is what you need.

Use mild soap (Liquid Woolite is good) and add some baking soda. Let soak for several hours (maybe turning machine on once in a while for a couple of minutes). Wash for only a few minutes and double rinse - add a cup of vinegar on second rinse.
Find a nice flat surface where sunshine will be for several hours and shape your afghan so it will dry in the proper shape.
This should work but if any yellow remains just pour water on the spot and let the sunshine (and even dew overnight) do the work for you.
I have had to do this many times over the years and it usually always worked.
Store in a cotton pillow case and never in plastic.