For 2023 returns, the "election" had to be done by making a payment by 11.15.23 (extended from 6.15. - the normal date).  If that was NOT done, the opportunity for 2023 is not available.  Period.  Full stop. The FTB offers no waiver or relief.

For 2024 returns, the "election" will need to be made by 6.15.2024 (absolute date, baring any disaster declaration) by making the required payment.

I *strongly* suggest you sign up for Spidell's (catax.com) message board for Ca information.

As to 'should' this be done for your client - that is dependent on many, many things.  Spidell offers numerous webinars on PTET; I took several of them when this SALT bypass option became available.