Does a particular codeshare flight ever stop being booked on United.com?

I have a voucher that I want to use on United.com for a codeshare flight on Air Canada. I have confirmed that I can use the voucher for the flight.

When I checked Kayak.com a few days ago the flight shows in the results as the carrier being United operated by AC.

Today, the flight appears in the results as the carrier being AC.

I checked the UAL website and it is still on the site as United operated by AC.

It made me think. Does a particular codeshare flight on United.com ever stop being booked on the website?

I want to make sure I can make use of the voucher.

I went on aircanada.com and checked seat availability for the site and there do seem to be fewer seats than there were a few days ago. As the flight becomes filled, is it possible that it is no longer booked on united.com?

The flight is for August 8th. I wanted to wait until May to book the flight.
The plane is an A319. There are 46 seats left. Is that a lot of seats available at this point? Is it possible the plane will be full by May?

Is there a good chance the that the fare price will fall by May or should I not even wait? I read that 3 months from a flight date can be a good time to buy a plane ticket.