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#11
It isn't. Like most advice on SSDs one sees on the internet it is advice from another decade that was most likely not true in the first place.
The SSD does it by itself. If you buy an enterprise class SSD it will generally have a higher percentage assigned to overprovisioning than a home one. This is to increase reliability.
This is invisible to you the user (or you the operating system). The SSD does not expose the full capacity.
There is an argument that running a SSD 99% full is detrimental but you won't be anyway. You'll be running 99% of the exposed capacity. In any case making a partition and leaving unallocated space is pointless. If you were really worried about it you should just make one partition and not fill it to 99%.
You could do better than read this Exploring the Relationship Between Spare Area and Performance Consistency in Modern SSDs