How do I add filters in Twenty-Twenty-Three without a functions.php?

I tested this just now, created functions.php in twenty-twenty-three and it does work.
I created two full site editing themes so far, and both have functions.php, so it definitely works.
My previous remark about supplying true argument to filter was incorrect, thanks to @Rup for pointing that out. add_filter parameter is always callable and __return_true is callable which points to function returning true.

So I would conclude, if it does not work it is likely plugin bug, or another bug in your code.

Also, you should create and use child theme, because with next theme update (which do happen!) your function.php will likely be deleted.

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