Both V and Nintendo have issued similar statements.
As we enter the "early season", the weather is getting hotter and hotter, and this year the global heat is particularly severe. Steam Deck is safe to run in these temperatures.
Valve states that the optimal operating temperature for the Steam Deck is 0° to 35° C. If the ambient temperature is higher than this, the Steam Deck may go into power saving mode. For the internal temperature of the device, it works well up to 100°C. If the internal temperature reaches 105°C, it will automatically shut down to protect the device.
Most electronic devices have difficulty performing well in excessively high ambient temperatures, and Nintendo issued a similar statement last week that ambient temperatures above 35°C may overheat the Switch, in addition to being careful to Don't block the exhaust and air intake vents. The device will automatically go into sleep mode when the temperature gets too high.
In these hot summer days, it's better to play games in an air-conditioned room.