Does iPhone battery life decrease significantly at 89°F?

No, an ambient temperature of 89°F (31.6°C) should not cause your iPhone’s battery time to get very reduced or decrease significantly.

According to Apple Support, the official safe operating range for an iPhone is 32°F to 95°F (0°C to 35°C). Because 89°F falls completely inside this safe window, it is not normal for the battery to experience drastic, sudden draining purely from the outside air temperature. 

No, there are other factors where you would see an increase in battery drain:

• Immediately after any update, specifically for about 4 days for the background processes to finish indexing the files.
• Battery Health is below 80% or close to it.
• Apps using a significant amount of battery or those using Background Activities when not using the app. Social Media apps are particularly known for this.