If you only want to visually hide the points outside the polygons, but still keep them, simply click next to `Enably symbol layer` to data driven override (list) icon <kbd>ɛ</kbd> > <kbd>Edit...</kbd> and insert this expression: `overlay_within ('polygons')` and replace `polygons` with the name of your polygon layer.

All points outside the polygons visually disappear, but are still there in the attribute table.

*Screenshot (click to enlarge): above, only points inside a polygon are shown, based on the expression; compare with the image below, same layer, but this time with no data driven override for `Enable symbol layer`:*
[![enter image description here][1]][1]


  [1]: https://i.sstatic.net/zWq85.png