LTO-8 was launched in 2018, although it did not become available to the channel until mid-2019 after Fuji and Sony settled their patent dispute. LTO-9's launch has been delayed twice, but it finally looks like LTO-9 drives will hit the market in late September.
Quantum had an LTO-9 pre-purchase program last year (and they have one now), so understandably the first batch will go to their cloud customers and those who pre-purchased it, so it probably will not be available to the channel immediately after launch.
With the native capacity of 18TB, the capacity of the LTO-9 media is 50% higher than the previous Gen 8 and not double what it normally is. Transfer rates are a little over 10% higher from 360 MB /s to 400 MB /s (Native rates). The compression ratio is still assumed to be 2.5:1, so the compressed capacity and transfer rates increase in the same proportion as the native rates.
The LTO organization's website mentions that the LTO-9 tape drive will have full read and write compatibility with LTO-8 tape media, but it does not mention whether it will also read LTO-7 media. Typically with LTOs, previous generation media is read and write compatible and second previous generation media is read compatible, with LTO-8 being the only exception. Also LTO-7 Type M (M8) media would be mostly incompatible ( LTO-8 drive will be the only drive that supports LTO-7 M8 media). LTO-7 M8 was used by owners of LTO-8 drives as an interim solution when LTO-8 media was unavailable due to a legal dispute between Sony and Fuji.
For long-term, cost-effective, and secure air-gap data storage, nothing works as well as LTO tapes. LTO-9 with a capacity of 18/45 TB will be essential for protecting massive unstructured data and it's about time!