I think those guidelines originated when LiON batteries were first adopted into wide use and are dated.

The recommendation to partially discharge and frequently recharge was, in part, to differentiate LiON batteries from the NiCADs which you want to fully discharge before recharging. However, you don't want to discharge your LiON batteries below 10% as that can be harmful. You also don't want to over-charge, or keep charging a fully charged LiON battery as that can lead to "battery bloat", which was a common complaint about Dell laptops a few years ago.

Charging technology has changed over the years. Now, most of the better chargers are Constant Current / Constant Voltage units which start charging at a Constant Current then switch to Constant Voltage when the battery approaches a full charge. That switch slows down the charging rate and helps minimize heat to protect the battery. When the battery reaches a certain charge level the Constant Current mode would be trying to force electrons into the battery faster than the battery can accept them and the excess electrons become heat and heat is bad for electronics. At a low charge level you're trying to pull electrons out of the battery faster than the battery can provide them which will damage the battery, although most devices will now shut-down when the charge level is too low.

I routinely store LiON batteries fully charged and ready for later use. If I didn't, I'd need to recharge them before I used them. But, the chargers are of the CC/CV variety and I don't leave them on the charger after it finishes the charge. I generally keep my laptop plugged into the charger when I'm using it because of the limited battery life but, as Mikiem mentioned, I adjust the laptop's power parameters to consider something less that 100% as fully charged so I don't over-charge the battery.

You can limit the charge to 50-60% on many laptops & iPhones, which manufacturers claim gives the battery a greater lifespan. You can also find plenty of articles online saying that prolongs battery life, but I've also read an article on ZDNET where the author followed that recommendation for two+ years with his new iPhone, and it didn't seem to make any difference compared to previous iPhones he'd owned. That's a little bit apples & oranges, because different year iPhones may have different typical battery lifespans, but it may or may not make any real difference. I asked Microsoft's Copilot and it said basically charging to 100% can create more heat & that heat may shorten the lifespan, but it listed Android Authority as the source several times, and many people claim the stuff they publish is often inaccurate. One disadvantage of limiting the charge that way on a laptop is it can throw the charge estimate off more than charging to 100%. The charge estimate you see is very much a guesstimate, which is calibrated by charging to 100% followed by discharging to 15-20%.

I keep hearing that the "butter zone" for lithium Ion batteries is 40-80 percent. That implies that it's "bad" to run them when fully charged. For one thing, once you start charging them, it's hard to stop at JUST 80%. Some products don't tell you the actual percentage of charge in the battery. Some instructions tell you to fully charge the battery before you use the product.

What's wrong with charging them all the way and letting them slip below 40% before charging?