when I delete a photo in my iPhone , will the deleted photo still be in the iCloud photo library

If you delete them from your iPhone, they will be deleted from icloud photo library as well. iCloud photo library is not an archive.

Import the photos to your computer before deleting.

Yes, storage sites like Dropbox would work

I use DollyDrive to archive / backup any photos and documents on my computers.

There is no limit to how long DollyDrive stores files that are deleted from a Mac computer while most other providers that I have used typically store deleted items for 30 days or less and one way or the other at some point that is not enough.

Dropbox will store some deleted material but is still more suited fro syncing across devices just like iCloud.