Iris Shader Compatibility
Immersive Portals mod roughly works with some Iris shaderpacks. Not all shaderpacks are compatible (Sildurs Vibrant shader and path tracing shaders are incompatible).

It's recommended to turn off settings such as TAA, Anti-Aliasing and Motion Blur in the shaderpack's settings.

The portal rendering with shader is not perfect because of technical difficulties. The Iris/OptiFine shaderpack standard is not designed to work with portal rendering. I already tried hard to hack the shader rendering process to make it work with portals.

Following things cannot be correctly rendered with shaders:

Semi-transparent things (e.g. stained glass) in front of the portal. (Due to that, all mirrors will cover the glass texture with shaders on)
Portal overlay
Fuse-view portals
Cross portal entity rendering
Temporal Anti Aliasing (TAA) when a portal is in view
Motion Blur when a portal is in view
World Curvature with portals
Other than that, effects like rain distortion will be abnormal with portals. And the portal rendering with shader does not work on some ancient videocards.

As I tested, BSL shader works mostly fine with Immersive Portals.