This is a Y tensioner based on these 2:
https://cults3d.com/en/3d-model/tool/ender-3-s1-s1-pro-s1-plus-y-axis-belt-tensioner
https://www.thingiverse.com/thing:7014002/remixes
The best of both worlds, super stable.
The author remixed this model.
It's a combination of the 2.