I am planning to put some MOC’s in the attic and I could see parts melting slightly and fusing together in the heat.
A friend of mine in Richmond had a small house and stored all of his Lego (brick and MOCs) in his attic for years without any noticeable harm. The attic was not temperature controlled. Richmond is know for it’s slightly warm summers (several months of 90+ and days of 100+ are not uncommon). He measured temps in the 125+ in his attic during the peak of the day. Obviously, YMMV.