GitHub sure doesn’t make it obvious, but you can filter issues in the the new Projects with an “or” operator in between conditions by joining terms with a comma:

In this screen shot I’m specifying that issues with the milestone “2023.23” OR “2023.24” should be shown by joining the term with a comma, like this

milestone:"2023.23","2023.24"

I don’t think the UI does this. I had to search it here https://github.com/orgs/community/discussions/4507, and then type the condition manually.