In my family, we have an antique christening gown.
It was made in 1928 from the lace of a wedding dress worn in 1889.
Needless to say, it has yellowed a bit over time.
My mother currently has it, and has wanted to clean it for YEARS!
But she doesn't want to do it, or be responsible if something happens if she gives it to a dry cleaner.
My niece will be wearing it at the beginning of November and will be the 21st person to wear it.
Why would you want to wipe out all the implied history of the yellowed wool?
I'd suggest you wash it and leave it be. If it has yellowed, well, it's showing that the shawl has been around for a good few years. That should be celebrated, not disguised, glossed over or destroyed. Much of the value of antiques is in the patina that time has left on well loved items.
I do feel strongly about people wrecking antiques by a nonsensical desire to make everything look 'new', 'fashionable' and most disgusting of all, 'trendy'.
If this shawl is precious, I'd hesitate to try any 'abrasive' treatment. I'd wash it in a specialised wool-wash very cool to freshen it and leave the ageing colouration to add to the obvious history and experience that this shawl possesses.
It would be a great shame to spoil anything so delicate.
My sons were christened in a very old 2ply wool shawl which had been crocheted by my grandmother for my mother's christening. It was quite yellowed but I was amazed by the comments of how lovely it was to see such an obviously old shawl being used. I washed it in wool wash but didn't worry about the discolouration.