Is the LIbreOffice extension still needed?

[dnaber (Daniel Naber)]

@GerryT

Thus, for grammar check on your computer (and not remote/cloud/internet), you require the extension.

Well, you can also run the LT server locally and point the extension to that server (see LanguageTool embedded HTTP Server | dev.languagetool.org and New LibreOffice 7.4 LanguageTool Remote Grammar Checker - #2 by dnaber).