kigmatzomat
Legend
We're not just talking about adventurers though, we're talking about how these high level spells would effect the setting. These high level spells aren't going to be common. Of course that doesn't mean they can't warp society, it's just that even the largest of cities is only going to be able to cast a few of them each year
Is it all society, or "adventurer society"? Per OP
....20th level just means that someone has arrived into the "real influence" tier of society, where the movers and shakers of society start to take an interest in you.
...if a fair number of people have consistent access to 9th level magic, what sort of implications would that have for how "adventurer society" acts?
This implies that there is a non-trivial number of people who are 17+ casters in each region. Those 9th level spells ought to be cast.
This does bring up probable trait of high level casters: they don't cast high level spells on request or on any schedule. They are noncomittal and then at some later point the person may wake up in a strange place with the caster asking "did you still want that leg Regenerated/child Resurrected?" Why? Because using powerful spell slots leaves the caster weaker and they wouldn't want to be able to let an assassin's helper just pay for a high level spell and know the caster can't use one of their biggest tools. Especially since the 8th level slot is almost always consumed by Mindblank.