Most believe that they're trying to prevent safety issues with overheating batteries. This update prevents the battery from being charged too much or too little.
If it is just that, they should have written so. Because what they wrote was that some devices may be affected from this optimization. Nothing general. And if this is really an issue, all those that now change to Lineage or Graphe will be an even bigger issue soon. Because in that case, Google knew, and did not say so.
Draining the battery from 100% to 0% in under an hour (as other commenters have mentioned) is the opposite of this, as that will necessarily dump a lot of heat.
No, it's not actually draining the battery faster. It's setting low level limits on maximum and minimum charge levels. (and also the charge rate, further controlling heat issues)
Some folks have hooked up devices between the phone and charge cable that do the coulomb counting (or similar), and (the one case I saw) showed that the phone is only taking in about 40% of the watt-hours that it was doing before the OTA update.
From what I've seen, no one can point to any actual incidents with these phones being dangerously hot. But the google FAQ did have a "Yes you can still take the pixel 4a on flights", reminiscent of the samsung phone issue several years ago...
