No they're not public domain until seventy years after the death of the author. However, depending on where the author posts them, you may be able to use them in certain ways. Any stories posted to this wiki come with preapproved rights, as follows:
You are free to:
Share — copy and redistribute the material in any medium or format for any purpose, even commercially.
Adapt — remix, transform, and build upon the material for any purpose, even commercially.
The licensor cannot revoke these freedoms as long as you follow the license terms.
Under the following terms:
Attribution — You must give appropriate credit , provide a link to the license, and indicate if changes were made . You may do so in any reasonable manner, but not in any way that suggests the licensor endorses you or your use.
ShareAlike — If you remix, transform, or build upon the material, you must distribute your contributions under the same license as the original.
No additional restrictions — You may not apply legal terms or technological measures that legally restrict others from doing anything the license permits.
This is why you should only post your own stories on this wiki, and not the works of other people. Otherwise you're giving away rights that aren't yours to give. Stories posted to other sites, such as creepypasta.com and reddit, do NOT have these rights attached to them, and you must seek the author's permission before using them for narrations, etc.
Source: https://creativecommons.org/licenses/by-sa/4.0/deed.en