The original Garmin Instinct was released back in October 2018 and it is one of the last watches to be launched before Garmin introduced their new Elevate HRM with PulseOX. It received very favourable reviews with its chunky Casio-like design and if often regarded as an affordable Fenix.
An eager-eyed Reddit user potted new firmware on Garmin servers which indicates that a Garmin Instinct 2 is in the works and will bring with it both the new heart rate sensor with PulseOX (like all the other new watches) and also solar charging which was first introduced with the Fenix 6 range.
The firmware for the Instinct 2 Solar is currently in Beta so I wouldn't expect a launch too soon. At the same time I would have thought it will launch earlier in the year than the original Instinct, Garmin launched the Fenix 6 earlier than expected too, so I think this adds up.
Currently solar on the Fenix adds about 10% to the estimated battery life. With the Instinct having a monochrome transflective memory-in-pixel (MIP) display it requires a lot less power than the Fenix equivalent, so it could make use of solar charging better than the Fenix.
With the addition of the PulseOx the new watch will also introduce various Firstbeat features such as Body Battery. It should also introduce optical heart rate monitoring for swimming.