It's available. Seems weird to decide based on the availability of the number versus the number.
I don't see result by searching "glycemic site:slimfast.com". There are some numbers posted on other websites (e.g. http://www.glycemicindex.com/foodSearch.php?num=1469&ak=deta... which looks credible), but compare to Soylent's disclosure (https://faq.soylent.com/hc/en-us/articles/212769503-Glycemic...) I'm more confident to choose Soylent instead of SlimFast.
Out of curiosity, I also searched GI/GL info on amplemeal.com and found nothing.