fast drop of battery health

My battery health is now just 87% for 11 months of usage.

Normal and no reason at all to be concerned.

The general "rule of thumb" is that Battery Health will decline an average of about 1% over longer time periods.

If the "rule of thumb" were a law, you might expect Battery Health to be at about 89% after 11 months, but it's not. Variations of a few percentage points up or down are expected. Remember that battery decline is also not linear. The Battery Health might drop 2-3% in a short period, then not drop at all for a few months.

You might also not be aware that Apple does not display tenths of a percent and always "rounds down" the displayed number. So, your battery might actually be at 87.9%, but this will be displayed as 87%.

Use Optimized Battery Charging and connect the phone to the power adapter at night and leave it connected all night.....every night.

All that you really need to rememberber is to replace the battery when it drops below 80% Maximum Capacity. That will be roughly about a year from now.

You should not be concerned. Most typically an iPhone's battery health doesn't decline linearly but does so in a volatile pattern. It can drop a few percent rapidly, it can also not drop over a few months, then drop rapidly. But normal decline for your model is 1% per month, so after 11 months you should be at the 89% range. Battery health is also rounded down a tenth so in reality 88%~ which appears to be normal.