What percentage of battery health is recommended for replacement?

Msraimo wrote:

Is 80% when performance gets throttled?

Not always. We don't know how an iPhone determines whether or not it should go in that mode, but there have been some screenshots showing the battery health under 80% while still displaying that peak performance is still enabled.

If you have any issues, one can use a USB power pack as a stopgap to extend run time until the battery can be replaced.

Once it reaches 80%, it should be replaced.

Apple recommends replacement at 80% (their standard for "depleted"), but you should be free to replace whenever you want for whatever reason you want as long as you pay for it.