flushing is for extreme problems. It has a use, just not at end of harvest. As Comic the Insult Dog would say, "Flushing is good ... for me to 💩💩💩 on!!"
If you don't feel like mixing some nutes the last few days, it also won't impact it much. your canopy should be a bit more lush heading into a 'flush' though.. if you do have a hankering to do it, i'd wait longer than typically suggested for your growing medium. your plant has already sucked the leaves down in a visible manner.
does outdoor weed have a problem? cause removing all nutrients from the soil around the plants is not a feasible option. This idea is an oversophistication of hobby growers. I've never met anyone that isn't a hobby gower and most seed producers only use methods that date back to the 1800s, so the whole industry is a bit behind the times, tbh. Not surprising given typical legality of it around world.
The point is, people without enough educational background have made a fair share of hypotheses that were never tested. that amounts to guessing. Over time it is inevitable more things like this will be outed as nonsense. Beliefs can be dangerous. popularity of a belief is only slightly correlated to reality, lol. What is popular among the peanut gallery is not a good bar to use in these contexts.