I am using preact@next and can't seem to find the memo function or the PureComponent class.
are they included in Preact? If not, what's the best way to use a "memo" functional component?
Both of those reside within preact/compat, not within the base.
As @pmkroeker said: These functions are part of the preact/compat export.
import { memo, PureComponent } from "Preact/compat"