Yes, it features a 3-stop Image Stabilizer (IS) system that allows for sharper handheld shots at slower shutter speeds.
Yes, it is compatible with Canon Mirrorless R-series cameras when used with an EF-to-EOS R Mount Adapter.
No, this lens is not officially weather-sealed. It is recommended to use a rain cover when shooting in wet or dusty conditions.
The Canon EF 70-300mm f/4-5.6 IS USM uses a 58mm front filter thread.