Following the end of "Arrow" earlier this year, Supergirl is set to end it's run on the CW with an extended sixth season composed of 20-episodes. While a lot of reasons are being thrown around, one noted reason is the show has been experiencing declining ratings (Something that led to the show being cancelled initially by CBS before CW picked up the show). Thus, a longer swan song season was opted for by the creative forces involved.
Shooting on the final season will begin next week in Vancouver after production for all of the CW's broadcast series for the 2020-2021 season was delayed by the ongoing COVID-19 pandemic. Not to mention, in this case of this show, lead star Melissa Benoist delayed it further with her pregnancy (She will actually joining the production later because of this).
So, if you're a fan of the show, prepare for the end of another corner of the Arrowverse. Stay tuned!