@kim1003 Research backed information about using Slippery Elm, for any condition, is very limited.
Here is one article I found in from the University of Wisconsin Integrative Medicine Department:
https://www.fammed.wisc.edu/files/webfm-uploads/documents/outreach/im/module_gerd_patient.pdf
Mayo Clinic, on the other hand is not so certain;
"Alternative medicine
Some complementary and alternative therapies, such as ginger, chamomile and slippery elm, may be recommended to treat GERD. However, none have been proved to treat GERD or reverse damage to the esophagus. Talk to a healthcare professional if you're considering taking alternative therapies to treat GERD."
https://www.mayoclinic.org/diseases-conditions/gerd/diagnosis-treatment/drc-20361959
Finally, a word of caution - Slippery Elm is primarily mucilage - a mucus-like, gooey substance, which can coat the lining of the stomach and also the intestine. If you are taking other medications, this may block them from being fully absorbed and lessen the effectiveness. So be sure to let your doctors know if you decide to try this.