Yeast pitching temp/ ferment temp

Specifically when using S-04 (or most other English Ale yeasts) they are very sensitive to temperature drops. Other strains might tolerate starting so high, but the cooling wort is likely to send an English strain into hibernation early.

While pitching S-04 at 80F may or may not effect viability; as the wort cools the yeast might go dormant. Most English ale yeasts ferment quickly and then they want to drop out (flocculate). If the temp of the wort is still >75F when things get started the yeast will sense the dropping temperature on the way to ambient. A drop in temp tends to signal English ale yeast to quit even if there is still sugar to ferment. For English Ale yeasts this can lead to stalled ferments. I have seen this many times myself.

I often pitch the yeast as soon as I reach the highest temperature that is supported (depending on the yeast it can be around 25°C to 28°C).

Many books and articles mention that the wort is most vulnerable when it is cooling down. The heat no longer protects the wort and other bacteria could contaminate it, so it is better to pitch the yeast at high temperature rather than waiting hours before reaching the perfect temperature. Honestly, I could not really tell the difference between a batch where I pitched at high or at a perfect temperature.

Of course the ideal is to get a wort chiller (or ice bath for smaller batches) so it does not take long to reach the perfect temperature. When you become good at brewing and want to improve, little things like this can make a small difference. Although sanitation and good raw materials are much more important in my opinion.

To answer the question, read the maximum temperature supported on the yeast package. Pitch at that temperature if you want, the off flavor will be minimal. Safale s-04 mentions a maximum of 25°C (77°F), so I would follow the instructions. If you think it would be too long to get the wort from 28°C to 25°C, I would think it is better to pitch at, lets say 27°C (close enough) rather than waiting one or two hours. The off flavors will be minimal and most will not notice the difference.

I do this all the time because I'm too lazy to wait for the last 10 degree or so when I cool my wort. I rehydrate my dry yeast in some sterile warm water or wort and after it's a nice slurry, I pitch it around 85f and never had a problem. The wort is usually cooled off before the beer really starts going. Or you could pitch some Norwegian Kviek it actually likes temperatures in the 80s and 90s. Good luck!

Actually, no, that is not good practice. Always pitch relatively cool, and either keep the temperature stable, or only let it rise, but not too much.

If yeast started at a certain temperature, then there is a big chance that it will stop fermenting if the temperature drops.

And 83F (28°C) is much too high to start. It will not kill your yeast, but make a whole lot of higher alcohols. And when the temperature drops, the yeast will probably stop early, and thus not ferment deep enough.