The variant cover of the two most recent editions of Supergirl pays tribute to past Supergirls.

I recognized the Supergirl on the cover of 18B (Feb 2018) as the Supergirl of the 1970s as she regularly appeared on and between covers of Adventure Comics.

And I know the Supergirl on the cover 19B (Mar 2018) as the Supergirl in the DC Animated series Justice League Unlimited (2004-2006).