Normally,we don't recommended to customize the suite nav with either CSS or JS.
If you still want, for modern experience sites, we can use the SPFx application extension to inject custom CSS on modern SharePoint pages to hide "SharePoint".
Refer to steps:
- Create a CSS file named custom.css with your customizations and upload it to “Style library” of the root style library(which can be found at https://[yourtenant].sharepoint.com/Style%20Library/Forms/AllItems.aspx.)
- Download or build the SPFx solution package from https://github.com/hugoabernier/react-application-injectcss.
- Upload and deploy the .sppkg file to tenant App catalog:
- Activate the app to one of your modern site via adding the app.
- After that, your custom CSS should work up.
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