The issue with storing records flat has already been brought up. Primarily, the increased potential for warpage and sleeve damage. This will depend to some extent on whether the record in question is a box set or not, generally a box set is more durable as to the contents. Easiest thing to do is just set the record on the platter and do a little spin, this should tell one if warping is present.
Records not stored vertically for decades
I’m looking to eventually get my vinyl rig back up n running and went to get my albums and found they’ve been lying flat probably for decades in two stacks of about 75 albums in each stack. Any chance they’re still in good shape, or what damage may have been done versus being stored vertically? They’ve been in a dry, temperature-controlled basement the whole time. No way to play them right now so just hoping they might still be ok. Thanks for thoughts.