In a development version of 0.4.15, the sneak glitch was disabled by default. This mod is an easy solution to re-enable the sneak glitch for your world.
And not just that! If you dislike the new sneak glitch, simply add `sneak_glitch.use_old_code = true` to your minetest.conf file and you will be able to use the old sneak behaviour again. Beware! This setting may lose its effect in Minetest 0.5.0 or later, as there hopefully will be a solution to clean up this mess.