DevOps builds alignment between Dev & Ops teams. So be it Waterfall / Agile / Hybrid projects, DevOps can fit in anywhere. The culture of collaborative responsibility has to be built among various teams who work in developing / maintaining the product/project/service.
Now DevSecOps is also being practised for working together with Security teams as well.
I have a slightly contrary view to my above senior colleagues. Devops will give you control on your deployment and ops pieces for any project hitting your production or UAT environment. Though I am coming from the Agile background, initial 10 years of my experience are from the waterfall so I can relate with your pain.
My simple but a bit longer answer to this. You may not extract the best value out of deploying DevOps in a waterfall setup,. Wait for it.. I have relevant points to state this.
1. As your waterfall is taking months to navigate through each phase, the automation of the CI / CD to enable DevOps will not be fully optimized.
2. Only when the testing stage is reached and you are deploying your code to SIT / UAT / PROD environments, the CI CD will play the role then.
3. This will ensure you have the right build going in for testing and no monkey business is conducted on an existing build, like all manual effort to prepare a build is now covered under automated CI/CD, this saves you a lot of headaches
4. but if your projects are long and each of the phases run into 6-8 months of effort then DevOps is not your way to go. I agree with Sergio, that you can patch your process by a simple and effective configuration management or versioning control system and you're done.