I cannot authoritatively answer whether your IP address is being stored with each post and/or where and for how long but I would assume yes. Basic analytics does as much.

If you are wanting to be anonymous on any site on the "Internet" and don't know what you are doing you are not being anonymous. Just assume everything you are doing as trackable.

Trust no one!

https://steemit.com/privacy.html

See #6, 7, 8 and 9