The method[1] currently has no scope defined. It uses $wgUser, and will eventually need to have its signature changed to require a user parameter. Until then, it should be made private.

https://www.mediawiki.org/wiki/Manual:Coding_conventions/PHP#Visibility says

Mark new code with proper visibility modifiers, including public if appropriate, but do not add visibility to existing code without first checking, testing and refactoring as required. It's generally a good idea to avoid visibility changes unless you're making changes to the function which would break old uses of it anyway.

Looking through every known use of getParserOptions[2] shows no calls outside of the class.

Since the plan is to make breaking changes (require a user parameter), I suggest it be made private now to avoid the introduction of any uses that will then need to be changed (i.e. to ensure that it only remains used within the class)

[1] https://gerrit.wikimedia.org/g/mediawiki/core/+/a50432f78c6986389fb75e20fc9ce1edf1d37015/includes/cache/MessageCache.php#202
[2] https://codesearch.wmflabs.org/search/?q=getParserOptions&i=nope&files=&repos=