I keep using LibreOffice for just the spell & grammar check, earlier it was MS Word.
Is there an offline version for English language that is multi-platform?
Basically an app like Windows Notepad with spell & grammar check.
Notes on the Mac has spelling and grammar checking. Most Mac apps have this. Goto the edit menu and there is a spelling and Grammar selection. You can turn these on by selecting options you want. I don't know how well these work. Since this seems to be a system wide option it works with Textedit too.
You can also get a free account with Grammarly. It runs in background but monitors everything you type.