If you try to use Fossil 1.37 — the last 1.x release — to clone a repo that has SHA-3 hashed artifacts in it, it says, "server returned an error - clone aborted". Since 1.37 pre-dates this feature, it can't give a more detailed diagnosis than that.
If you have an old clone made from before the transition and try to update it, I'm not sure what it says, since I don't have any of those around any more. It has, after all, been three years since Fossil began to move on this problem, so that it's largely a past issue for us now.
This transition time was indeed annoying for us over in Fossil land, but Git's going to have to go through a transition like this, too. The question isn't whether but how long we'll have to wait for it to begin and how long it'll take to complete.
In large measure, you actually can't, since there's a good chance those repos are behind HTTPS-only these days, and those versions of Git will be linked to ancient versions of OpenSSL that won't even talk to modern TLS implementations, the two being unable to agree on a common ciphersuite.
Beyond about 10 years, you usually end up freezing old binaries in place along with old data in order to continue manipulating it anyway.
If you have an old clone made from before the transition and try to update it, I'm not sure what it says, since I don't have any of those around any more. It has, after all, been three years since Fossil began to move on this problem, so that it's largely a past issue for us now.
This transition time was indeed annoying for us over in Fossil land, but Git's going to have to go through a transition like this, too. The question isn't whether but how long we'll have to wait for it to begin and how long it'll take to complete.