Namespaces in logseq are as simple as using a slash in the page’s title. If you have worked with namespaces in a logseq version, which is lower than v0.8.9, you should migrate your existing pages according to the “New file name rules”. These rules are mentioned almost at the end of release note. Otherwise you might notice some weird behaviour with the pages in a namespace.
The migration via GUI can be found in logseq: Settings → Advanced → File name format and there is an Edit button, which should be clicked and the instructions in the popup window need to be followed. In your logseq graph this will rename all pages with a slash, where the slash will be replace by three underscores.