Bottom bracket cable guide
 

Is it recommended to replace the plastic bottom bracket cable guide on old bikes?
My bike is from 1985/1986, so 32 years old.

The piece looks ok. It doesn't have the screw as in the ones I see on internet.
There is a hole on the frame (around 7.8mm) and you just push it to it.

No. Not unless it's no longer guiding your cable. If it ain't broke....

In your case, you will have some trouble finding another guide that fits that hole in your frame anyway.

My Bianchi (1982 model, 1981 production) and my son's Peugeot (1980) both have a similar setup, and I have no plans to replace either guide. If I did, I would be inclined to run a couple of short pieces of cable housing under the BB.

Thank you - yes there is no easy new replacement from what I looked on internet.
Thank you will use it care, so don't break down.

Feature: that benefits by staying open to let condensation moisture continue dripping out the bottom ..

That's actually very cool. On my frame with brazed on steel cable guides I use a short section of housing liner ($7 will get you a lifetime supply) to minimize the friction. If you have any problems with your cable guide (or even if you just want to protect the guide from wear) you could try that.