ValveSneak is a very simple mod for 1.16.5-1.21.8 (or later) that makes Sprinting behave like it does in some Valve games, like Counter Strike and Portal.
By default, you're sprinting. But if you press the configurable "Walk" button, you start walking normally.
For an authentic experience, set the Walk key to Left Shift, Sneak to Left Control and unbind Sprinting.