Is it ok to stack LPs horizontally?

overall, I would say its not a good idea.
Records are not an even thickness, the label area stands proud and so does the outer edge, it has a thickening or bead designed to guide the stylus into the start groove, it also helps in the manufacturing process.
While stood "almost" vertical is not ideal, I think its better than the combined weight of many pressing down on those at the bottom of the horizontal pile.
Though you could argue, that in the thick sleeve, this can be accommodated.
I think they are stored vertically, purely for ease of use. If you have a horizontal stack, its difficult to pull one from near the bottom and worse trying to return it to the same location.

I have a different experience with horizontal stacked storage of LP’s. While I do not recommend horizontal storage it may not be as risky as some people make it out to be.

I recently cleaned out a room in a relatives house where several hundred albums had been stored for more than thirty years.  About half these albums were stacked horizontally and NONE of them are warped.  
 

This room was not heated or cooled and was basically left to the whims of the ambient weather for decades, sometimes it would be 40F and sometimes it could be 90F.  No mold, no rot, just dust and some fading due to ambient light.

Your mileage may vary.