2018 IBC code requirement for tempered glass windows (safety glass)
No, it is not acceptable to use non-tempered glass, because the “seat” area is within 18” of the floor. 

The Code doesn’t say the window sill must be directly above the walking surface or if it could be recessed (like a window seat), etc.

Plus, you don’t want to put family and friends in jeopardy by having them next to such a hazardous situation. (What If a child walked along the seat?)

Btw, this code requirement requires the homeowner to change immediately. It is not “grandfathered” in as compliant. (There are 2 items in the Code that require the homeowner to up grade immediately: 1) tempered glass, and 2) smoke detectors.)