According to page 11 of Pub 560 under the heading, "tax treatment of contributions", contributions that you make to a SIMPLE IRA are excluded from the wages reported on your W-2 Form so they are NOT part of your taxable income. However, the contributions are still subject to social security and medicare tax. https://www.irs.gov/pub/irs-pdf/p560.pdf
Additionally, according to page 48 of Pub 590A, under the heading, "eligible contributions", these contributions are considered eligible contributions for the retirement savings contributions credit if you meet all of the requirements to claim the credit. https://www.irs.gov/pub/irs-pdf/p590a.pdf
Please refer to the following FAQ to review these requirements. https://ttlc.intuit.com/replies/3301263