trashaccount That wouldn't provide plausible deniability.
Until the OS shuts down or reboots after wiping what's needed to derive any of the encryption keys again, it can still access all the data. It still has access to all the data it did before wiping other than the stuff directly related to deriving the encryption keys which it doesn't need beyond trying to log in, change the password, etc. Rebooting or shutting down after wiping is a crucial part of it. That gets rid of the current OS access to the data and the data it had in memory. It's possible to implement wiping of specific keys and data in memory but it doesn't really compare to doing a reboot.
Wiping only secondary users without a reboot is possible but wiping secondary users will not be deniable. You would still be better off having a reboot after doing it to get remnants out of memory where it wasn't wiped by our page allocator zeroing and the standard implementation of purging keys from profiles that are put at rest or deleted. It's not strictly necessary but the reboot makes sure that leftover remnants despite having kernel page/slab zeroing, zero-on-free in userspace, etc. don't stay around. Bear in mind a lot of the core system processes handle more than one user including system_server and SystemUI. It's possible to clear everything that's no longer used but that doesn't mean a bunch of traces of what was being done in the secondary user don't linger around. Most will be gone, sure, but a reboot afterwards is best. Would you want to take a chance with this if you were depending on it? Probably not.