Do you guys have any details on the problem. “Too new” sounds weird. Does that version of fastboot have a regression that is known upstream, or are the files provided by sailfish maybe in fact somehow “too old” for a modern fastboot? After all other fast moving distros will be there soon enough
I suspect it has something to do with Android breaking adb/fastboot compatibility with older versions of itself or older Android images (like SFOS uses).
Always remember: all Google wants is for people to keep buing Android phones after security updates run out.