I have an application that uses v2 addresses-- if both ends (the hidden-service client and the hidden-service server) continue to use older tor client software (that "supports v2 addresses"), will they be able to work indefinitely? Presumably the actual implementation of the hidden-service resolution is in the network, right? Are there plans to turn that off in the network?
When new versions of the tor client stop supporting v2 addresses, will old clients still be able to access v2 addresses?
Yes, in order to run/access a v2 service, you need support from the service, client, and network. Even if you keep the client and service running old versions of tor, the relays in the network will eventually update, making the v2 service inaccessible.
The Tor Project gives the following timeline:
September 15th, 2020
0.4.4.x: Tor will start warning onion service operators and clients that v2 is deprecated and will be obsolete in version 0.4.6.
July 15th, 2021
0.4.6.x: Tor will no longer support v2 and support will be removed from the code base.
October 15th, 2021
We will release new Tor client stable versions for all supported series that will disable v2.
It's unclear from this when an official release will be published for relay operators that removes this v2 support, but a comment on the post says:
The tor 0.4.6 release (July 15th 2021) will prevent new v2 to be created. Once Tor Browser releases with that tor version, yes you also won't be able to reach v2 address as a tor client.
By October of that year, we plan to release new tor maintained stable version that remove v2 support meaning that from that point on, progressively the network will upgrade and v2 address won't be reachable.
This seems to say that the network itself will stop supporting v2 services as relays start updating in October 2021.