This has been posted in the weekly update since like July and all SMs were supposed to have the conversation with their partners about it back in August/September. But yes, you need to have 150% availability of the hours you want to work with the minimum being 12 hours a week (that's where they get the 18 hours from). Depending on where you work, that can be as little as two shifts a week. So if you want to work 20 hours a week, your availability needs to have at least 30 hours open to accommodate business needs.

However, if you're scheduled the bare minimum (12 hours) and call out without vacation or sick time, you risk being let go bc you won't make the minimum required amount of hours worked.

That really sucks that none of your leaders told you about this because this isn't new news. DMs were even given lists of partners that were at risk of being let go due to this policy so that SMs could proactively have talks and create contingency plans. There are some cases where an exception can be made, but again, your leaders should have been communicating with you.

Also, if you have open availability and your SM is cutting your hours due to this policy, absolutely question and challenge them. I think it's immoral and frankly bad scheduling if your SM can't make it work when they've literally had two months to prepare and practice this new requirement. Source: am an ASM who has been making schedules and was able to accommodate and support all partners in my store through this change.

Yes, and I believe we have to provide at least 18 hours of availability

Yes, you have too have 150% availability to work what you want, with a minimum of 12 hours worked.