Not taxable. It doesn't go anywhere. 

It was paid based upon timely filed 2020 returns. You snooze. You loose. 

CUTE - - -If you filed ON time and QUALIFIED but did NOT receive, how do you receive the CA stimulus

payment if you can NOT apply for it on your 2021 CA return?

EJO3,

The stimulus is taken as a tax credit on the 2021 return. See the snip.

This is copyright material from Spidell. Spidell offer very informative classes and are very popular with CA tax preparers.  www.caltax.com is THE best source for California tax questions.  

@Strongsilence-CPA The question was about a California Stimulus https://www.ftb.ca.gov/about-ftb/newsroom/golden-state-stimulus/help.html

@EJO3 If your client did not receive the stimulus, you should contact the Practitioner Hotline on their behalf, assuming you are a third party designee. https://www.ftb.ca.gov/about-ftb/newsroom/tax-news/september-2018/third-party-designee-and-implied-c....

You might also look for "stimulus" in the 2021 FTB 540 instructions.