Is it ok to enquire the editor about the accceptance chance of a paper which is under review for 1.5 years
I don't think the editor could confirm the acceptance chance without communicating with the referee. Perhaps you could ask the editor to expedite the process by asking the referee to cut short the report and submit it immediately if it's going to be negative.

You might not wish to do this if you want a more thorough report in the case of rejection.

You could alternatively ask the editor to send a reminder to the referee,  whether the referee agreed to return a report before a particular date, and confirm that they had not lost contact with the referee (since a long time had passed).