Correction, it is backwards compatible for one tape generation. (So your -9 drive will work with -8 tapes with no problem, but you would be out of luck for -7)
That being said though, tape generations advance infrequently enough that this would never really be a problem. And if it is a problem, the secondary market is absolutely loaded with older gen drives going all the way back to LTO 1 if you really need it.
Unfortunately, LTO is not truly backwards-compatible, not even for reading. https://en.wikipedia.org/wiki/Linear_Tape-Open#Compatibility