Does the entry now say it’s not installed for the current user? If so …
Edit: I just tried this myself and I could uninstall OpenKeychain without a problem. The App entry is gone, and the account it had in Settings - Accounts is gone, too. The OpenKeychain entry in Settings is still there, but Settings crashes when I tap on that.
Are you sure you uninstalled the package org.sufficientlysecure.keychain, and not perhaps com.android.keychain, which would be something different?