Should I keep my MacBook Air plugged into its charger whenever I can?

Yes.

With 10.15.5, Apple introduced Battery Health Management, and your Mac will run on battery occasionally even when connected to AC power, to maintain battery health.

+1 with neuroanatomist on this one.

does this include a MacBook 2010?

i have 2, one kept plugged in running Mojave-battery fine

the other always unplugged running Mountain Lion

the MtLion seems to lose battery % when i turn on the MacBook.

sometimes a lot were 8% and sometimes 92% are available

which is a battery- settings i forget to configure before turning off

thanks in advance.