Python 3 is the current version but the group which maintains OWFS have not updated their python libraries for a while so until they decide to release a new version of the python3-ow library it will not be available to download using apt-get. The upstream group in this case would be the Debian developers who maintain the Debian Linux distribution on which Raspbian Linux is based. As they decided to remove python3-ow from their apt repositories it also vanished from the Raspbian repository.
There are other methods you can use to access the 1 Wire Pi Plus and connected devices that does not involve python3-ow.
If you have owfs installed you can access your 1 wire devices through the file system in the /mnt/1wire folder.
Inside the folder will be a list of devices, for example DS28B20 temperature sensors will have a name starting with "28." followed by their unique identifier. To access the temperature of a sensor you open the temperature file inside the folder for that sensor "/mnt/1wire/28.0844C4030000/temperature".
To use the 1 wire devices in Python 3 the easiest method is to open the file and read the contents. The code below shows a simple example for opening the temperature file from a DS28B20 and reading the temperature from the device every 10 seconds.
device = "/mnt/1wire/28.0844C4030000/temperature"
temperature = open(device, "r")
if __name__ == "__main__":
OWFS caches the values from each sensor and updates it periodically so you may get the same value returned several times before it updates. If you need to access the current value from a sensor there is a /mnt/1wire/uncached folder which will fetch the data directly from the 1 wire devices when you open the file, for example "/mnt/1wire/uncached/28.0844C4030000/temperature".