Anything suspension should be replaced in pairs. I don't think Toyota would bother documenting something like this when there are so many variables. You are just wasting labor money and creating the next failure point by replacing one.
When I break them I tend to only break one at a time, so I replace one at a time. However, I recently broke a link for the third time and removed the bar altogether. No major complaints. Can't drive it like a Supra anymore though...
Also, you don’t want one breaking off and damaging the CV boots. Front sway bar links are way cheaper then replacing a CV boot.
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2003 V6 BLACK SR5 4X4-Runner
OB #1334 https://www.instagram.com/ob_1334/?hl=en
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I've found that most things wear out in pairs anyway. And if you're going through the trouble of doing one it does make sense to do the other... however, this rule only applies, for me, when doing something that's just "had it's day and needs replaced"
Things that I broke... because of some change I made or just me hammering on it too hard, I would likely only replace the failed part.
For endlinks, it would depend on why it failed. End of life, replace both. If broke due to abuse, just replace the one that broke.
If broken several... may want to re-access the bigger picture.
Keros spelled out a reasonable approach. The swaybar is a spring, similar in function to a torsion bar. It's purpose is to transfer force from one wheel to its companion on the other side of the vehicle. This means that as one endlink has upward force applied to it during cornering, the opposite side has downward (opposite) force applied to it. The endlinks work as a team.
So, as Keros described, for normal wear I replace both endlinks. If one side is failing for some other reason e.g. being struck by a rock during off-roading, the broken one gets replaced...and I'd investigate armor options to protect it.
If the swaybar is bent, it will preload both endlinks (in opposite directions) and accelerate wear. It may be necessary to remove the swaybar and check it for straightness on a level surface.