The next revision of the ChampCar Endurance Series BCCR will include a statement about the helmet SNELL ratings. ChampCar Endurance Series will extend out the currently accepted SA2010 and SA2015 helmets to January 1, 2022.
After that only SA2015 and SA2020 helmets will be accepted for driving use by ChampCar Endurance Series.

per SNELL Heads Up Letter, issue 72 – June 17, 2019 (pdf)

“SA2020 Final
A2020 was published in its final form last month.
It and an accompanying cover are posted on the
Snell website. Testing is already underway but the
first helmets certified to SA2020 will not be available
for sale or use until October 2020.

A2020 is more compatible with the requirements
of FIA 8859-2015 than SA2015 had been. The
impact test requirements are now generally more
demanding in terms of either severity or attenuation.
The differences with SA2015, however, are slight.
Snell will continue to recommend SA2015 headgear
in good condition to drivers and to racing officials.