Are stars expected to become dimmer before a supernova? With the recent news about the "fainting" of Betelgeuse and the speculation that this might be a precursor to a supernova, I'm wondering if there is any theoretical/observational basis for this interpretation, or whether this is a case of reading too much into a greater-than-average change in an intrinsically variable star. Why would a star become dimmer in the run-up to a supernova?
The connection between the dimming and a putative supernova relies on the interpretation that the decrease in luminosity may be due to circumstellar material, ejected in the years/decades/centuries immediately preceding a supernova. There are several mechanisms that could lead to this sort of mass loss (see slides 24-25), including


gravity-wave driven envelope loss (in red supergiants, during neon/oxygen core burning)
pulsations due to pair-instability in the days to decades prior to the supernova
turbulent eruptions à la luminous blue variables


These then lead to Type IIn supernovae, with narrow lines arising from interactions with the previously ejected circumstellar material. If the dimming is due to extinction by circumstellar dust from eruptions due to any of these mechanisms, it could mean that a supernova is imminent on a timescale of days to years.