What tone curve does Photoshop use for sRGB?

First of all, the premise for the question is faulty. Any tone curve will be remapped through standard color management, so that the net visual result on screen is linear. It's a standard profile conversion from the document profile into the monitor profile. Whether it's sRGB, gamma 2.2 Adobe RGB, or gamma 1.8 ProPhoto, it will all be displayed correctly on screen. You don't "see" the document profile tone curve.

Without color management, however, the document's native encoded tone curve is sent directly to screen uncorrected.

The sRGB specification is a custom and irregular tone curve, not a regular gamma function. Among other things, it has a flat "toe" section near black. The reason for this is that sRGB was originally specified as a description of a CRT monitor.  It was, in a sense, the seed for modern color management.

The sRGB tone curve is also used in Apple's Display P3, and Photoshop's Image P3 (those two are identical).

Modern LCD panels don't behave quite like the CRT monitors did, and so it's a bit less useful. But it has been close enough to survive to this day.