No, this lens does not feature a weather-sealing gasket at the mount. It is recommended to use a rain cover in wet conditions.
No, the EF 135mm f/2L USM does not have built-in Image Stabilization. High shutter speeds or a tripod are recommended for sharpest results.
Yes, it is fully compatible with Canon mirrorless cameras like the R5, R6, and R7 when used with an EF-to-RF mount adapter.
The Canon EF 135mm f/2L USM uses standard 72mm screw-in filters.