I once read somewhere that uc schools view your first semester grades of senior year before accepting you. And so if you were to get a D/F in the first semester, they won’t rescind you if it was in first semester, because they already saw it before accepting you. Is this true? Thank you
Not really.
You just report them what your classes are. Because deadline is around November 30 and your first semester grades come way later. But in a lot of cases, they ask for grades. Senior year isn’t as important as Junior year. I would say just to “focus focus” on SAT, EC, Scholarships and Admission process 
UC’s do not see your 1st semester Senior grades. The could ask for a Mid-year transcript if they send a supplemental. If you get a D/F 1st or 2nd semester Senior year, you are required to notify the UC’s before sending in your final transcripts.
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Freshmen: If you change schools, add or drop a course, or fail to earn a C or better in a course after you submit your application, you must notify the UC Application Center by email or postal mail. Your correspondence must include your name, UC Application ID number and your signature (if you mail a letter), and will be available to all the campuses to which you applied.
If you receive an offer of admission, be sure to confirm with the campus admission office that they are aware of the deficient grade or schedule changes. Although you must notify the campus of these changes, we cannot guarantee that a campus can go back to review an application after correspondence has been received. Make sure to save a copy of your correspondence just in case.
Email: docs@applyUCsupport.net
Postal mail:
UC Application Center
P.O. Box 1432
Bakersfield, CA 93302
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It is up to each UC to determine how a D or F will impact your admission decision. You would only get rescinded if they accepted you in the first place so you could be placed on the reject file prior to admission decisions in March.
in most cases, they don’t.
I do not know the answer to your question, but a recent poster got rescinded from UCI with a similar situation. He got a D in an A-G course first semester, and seems to have kept a 3.0 overall for senior year, but not a 3.0 each semester separately. At one point they said the D was ok, but nonetheless he was rescinded. Try to find his thread ,and see what you think. Seems, though, the very important part is that you notify/ied them when you receive/d the D, and that you have proof thereof.
Adding: it will matter what class it is/was. Is it a required course?
Just to add to the above post, another poster recently was rescinded from CSU Fullerton for a D in Senior English 1st semester, so there is always a chance your acceptance could be rescinded so make sure you stay on course in Senior year.
The scary part of the policy about no D/F grades in senior year is that the universities have the option to rescind admission. It appears that whether they do so or not can be affected by numerous factors – whether it’s an a-g course, whether it’s a course related to the major – and I also wonder whether they are more likely to rescind admission if they got a larger first-year class than anticipated. Some universities get more SIRs than expected.