to permanently and Globally [system wide] turn it OFF you have to do it via terminal, using defaults command
By "system wide" I mean: Will be turned-off for all applications, for the current user, (which will execute the command below). It will NOT affect (will not turn it off) for other users which may exist on the same MacOS machine.
Lets do it:
Open Termina.app, and copy/paste these 2 commands:
defaults write NSGlobalDomain NSAutomaticSpellingCorrectionEnabled -bool false
defaults write NSGlobalDomain NSAutomaticTextCompletionEnabled -bool false
After it, logoff and logon again, or reboot.
After logging in, those options will be disabled by default for every MacOS app, and if you wish, you can enable it individually for each app where it is necessary. (basically the behaviour will be the opposite of the behaviour you described on your question.)
How to Revert it back to Original Behaviour:
If you need/want to revert the behaviour back to the original, change the last command line word "false" to "true", execute both commands and log-off/on again.*
Example to revert back to original behaviour:
defaults write NSGlobalDomain NSAutomaticSpellingCorrectionEnabled -bool true
defaults write NSGlobalDomain NSAutomaticTextCompletionEnabled -bool true
------ ** The Answer for OP finishes here - But I will expand it with more related information below** -----
PS: Extending this answer a little, which may be helpful for others. There are 6 variations which can be explored/configured, to fine-configure those grammar settings:
Just use the same syntax as the answer, substituting the command for anyone of those 6 commands, and using True or False as the answer above instructed to activate/deactivate each one.