  Flat-spectrum radio quasars (FSRQs) whose brightness is dominated by a
relativistically beamed core, are frequently found in 'blazar state' commonly
inferred from a high optical polarization ( > 3%), and/or a large continuum
variability. Here we use these two prime optical markers to investigate
continuance of an FSRQ in blazar (or non-blazar) state over an exceptionally
long time baseline spanning 4 decades. Our basic sample is a well-defined,
unbiased set of 80 FSRQs whose blazar state stood confirmed during 1980s from
optical polarimetry. Four decades later, blazar state of each FSRQ is
ascertained here from variability of their optical light-curves of typical
duration $\sim$ 3.5 years, a low noise (rms $\sim$ 2%) and good cadence ($\sim$
3 days), obtained under the Zwicky Transient Facility project ongoing since
2018. For about 40% of these FSRQs, blazar state could be ascertained
additionally from the opto-polarimetric survey RoboPol (2013-2017). From both
these databases it is found that only $\sim$ 10% of the FSRQs have undergone a
blazar $\leftrightarrow$ non-blazar state transition over the past 3 - 4
decades. This reinforces the case for a long-term stability of blazar state in
individual FSRQs, despite their state fluctuating more commonly on year-like
time scales.
