Faculty TT verbal offer made, but no written offer (it's been about 10 business days)
A “verbal offer” is not an offer. in the US, any self-respecting institution would at the very least document their desire to extend you an offer in an email sent in very close proximity (same day, or next day) to the “verbal offer”. The actual offer including a precise salary anount and other hiring package information might take a couple of weeks after that, and official, final approval (typically by a university provost or other high-ranking official) might even take a few months, although that part is usually considered a rubber stamp that’s certain enough to happen that people will move across the country and even buy a house before it happens. But do not let anyone mislead you into thinking you have any sort of offer until something is, at the very least, documented in writing.
It is encouraging that someone says they want to hire you. I hope all will end well and that this is nothing more than an example of a mildly incompetent bureaucracy. But yes, they can change their mind (I have seen it happen), and yes, it is a red flag of sorts, or at least a yellow one. I advise you not to assume anything and to proceed with all other interviews as scheduled until your offer is extended in writing, with precise salary information. Good luck!