iOS 8: Prevent Safari Mobile from reloading tabs by default
As you have read, this is down to memory management. iOS will remove background tabs from memory when that memory is required for something else. The device you're using is more significant than the version of iOS. An iPhone 6 with 1GB of RAM will close tabs more aggressively than an XS with 4GB.

As others have mentioned in the comments, iOS keeps a thumbnail of the page in memory so that you know which tab is which. This thumbnail requires much less memory than a full web page rendering context.

Reclaiming memory used by inactive apps/tabs is actually a feature. You wouldn't wan't an app to fail to launch because your phone is out of memory, so iOS automatically makes space for it. For the same reason it's not necessary to "close" apps, as iOS will remove apps from memory when necessary.

Apple doesn't include RAM in the specs published for iOS devices, but wikipedia has a handy summary of iOS device specs.