At least in California, state funding to the UC system has definitely decreased if adjusted for inflation and population growth. Adjusted only for inflation, it's decreased in some time frames and is flat in other time frames. Here are a few years, all in inflation-adjusted 2019$:

    1970: $2.2b
    1980: $3.2b
    1990: $4.0b
    2000: $4.6b
    2010: $3.5b
    2015: $3.2b

So in the inflation-adjusted numbers, there's been a decrease since 2000 (and 1990), but flat compared to 1980. The state population has increased from 24 million in 1980 to 39 million today though, and state law mandates that UC expand in line with population growth (they're required to accept the top 12.5% of California high school graduates). So per-student funding has declined by more than $10k in 2019$, compared to 1980. In-state tuition has risen by about the same amount over the same period, making up the difference.