How good is 300 4 L IS for R6 ?

I see pretty interesting options on local second hand market, and i like this lens on paper, it seems reasonable for me, but it is clear to me that this is an old lens. I think i roughly understand what to expect from autofocus, based on experience with some lenses from this era (but i am note sure about tracking performances), but i had no experience at all with early IS systems. I just don't know how good or bad it is. Maybe it even worse, than 300 4 L "not IS" with IBIS, i just don't know.

Have you experience with this lens on modern bodies ? What you can say about IS, AF and in general ?

Thank you.

I have only used the EF 300/4 L and EF 300/2.8 L, both non-IS, and both on the 5DSR.

The f4 is almost as sharp as the f2.8, and AF appears to be very similar.  AF on the f2.8 with EF 1.4x seems a bit better than the f4 and EF 1.4x.  I often used both for tracking/panning race cars and motorcycles at high speed with great success.

The 5DSR autofocus should be well behind the R6, so you should see improvements beyond my experience.  The RF bodies are said to improve AF of all EF lenses.

The IS lens elements, of the 300/4 L, are said to have reduced IQ marginally  Personally I have no use for IS, so IBIS may in fact be good enough for your purposes, with the non-IS.

I suspect that with the age of the EF 300L f4, you will not get the full frames per second with the R6, if that matters to you.  There is a list of EF lenses that is compatible with the full 12 FPS speed and MS.  ES shutter will be OK