How effective is R6's IBIS with EF Lenses?

ArtHeals wrote:

I use a couple of Canon EF portrait lenses (135/2L & 85/1.8 USM) with EOS R and I don't have any major problems as i keep the shutter speed around 1/200 most of the time. But I was wondering how effective is the IBIS on R6 with these EF lenses. Is it still 5-axis with EF lenses or downgrades to 3-axis?

Some used R6's are popping up here and there and I am contemplating if its going to benefit my STILL images in any way - (No interest in video)?

Thanks in advance

I have less interest in video than still pictures but the R6 is a decent camera for video because it produces superior 4K/60 and 4K/30 compared to the same settings on the EOS R5.  In addistion, it apparently produces the same quality with 4K/60 HQ settings as the R5.  So unless slowmo or 8K is essential, the R6 video holds up just fine.   Your EF 135mm f/2 lens works real nice on the EOS R6.  Some images directly below of this combo...
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EOS 135mm f/2 USM lens on the EOS R6

Autofocus is fairly remarkable with this combination.

Eye-Detect AF (sent to "Animal Priority") works just fine as well with this lens.

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Some RF lenses with NO Optical Image Stabilizer (OIS) inside the lens (eg RF 85mm f/1.2L USM) get the whole 8-Stops of Image Stabilization from the IBIS mechanism.  EF lenses will also benefit although the EF lenses benefit to a lesser degree.
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But EF lenses like the EF 100-400mmL II lens get to combine their existing OIS with the IBIS mechanism inside the camera to get more/better stabilization than the lens alone can offer.  I managed to shoot at 1/40 sec with the EF 100mm L Macro lens although I was keeping quite steady when I took the shot.  I've shot at 1/4 sec with the RF 85mmL lens which has no OIS.  The EF 24mmL, EF 50mmL and the EF 135mmL lenses also performed better than expected on the EOS R6.  But if you want to know exactly how much stabilization you get with these non-stabilized lenses on the R6, I don't have immediate answers on hand so you might need to do some research online or send an email to Canon.
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Just a pleasantly sharp image with the lens pointed down... which should have impacted the ability for both OIS and IBIS to function properly.

EOS R6 - 1/30 sec with the EF 100mm f/2.8L Macro lens

EOS R6 - 1/40 sec from the EF 100mm f/2.8L Macro lens

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I was impressed with the added IBIS effect on the EF lenses I've used although most of the time you don't notice it unless you're thinking about it at the time.  Now I took a shot last Thursday of a Tiger at the zoo using the EOS R6 + EF 100-400mmL II + EF 1.4x III Extender and out of 10 shots, one was slightly blurred.  But in retrospect, this was the last picture in the set, the animal was moving and I was shooting at 200mm whilst physically moving myself to re-frame the shot.  It was my error although quite a lot was happening at the time and I was pressing two buttons repeatedly and simultaneously to get the Eye-Detect AF to ignore the stripes and find the Tiger's eyes.  Some people would look at this very-slightly burred image (it's only visible if you enlarge the picture) and be highly critical of the results.  Others would find it completely acceptable. All the other pictures in the set were otherwise very sharp indeed.
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Most of the time you will be watching your shutter speeds to keep them within a safe range.  But it's nice to know that the IBIS mechanism will apply something of a safety measure to allow slower shutter speeds.  Personally, I'm more concerned about the potential for Shutter Shock with lower shutter speeds.  With the EOS R6 it's said to be a possibility with images taken under 1/160 sec and with the EOS R5 it's said to be likely with images taken at 1/250 sec or under.  I haven't noticed any issues with the R6 at lower speeds though.  And the sensitivity with higher ISO settings with less noise ensures it's easy to keep at a decent shutter speed.
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A motivating reason to be considering using an R6 over the original EOS R would be the more advanced Auto Focus with Eye-Detect.  It's remarkable and very useful for portrait shooting.  Another reason would be the better lowlight performance (-6.5EV compared to -3EV) with less noise at higher ISO settings... although my EOS ra (which is a Canon-modified EOS R) also has decent low-noise performance so I'm assuming the R is much the same.