I updated to PHP 7.4.13 locally.
Unfortunately arc diff outputs an error:
[2020-12-24 07:55:56] ERROR 8192: implode(): Passing glue string after array is deprecated. Swap the parameters at [/usr/share/php/libphutil/src/utils/utils.php:1706] arcanist(), phutil() #0 implode(array, string) called at [<phutil>/src/utils/utils.php:1706] #1 phutil_build_http_querystring_from_pairs(array) called at [<phutil>/src/utils/utils.php:1678] #2 phutil_build_http_querystring(array) called at [<phutil>/src/future/http/HTTPSFuture.php:656] #3 HTTPSFuture::formatRequestDataForCURL() called at [<phutil>/src/future/http/HTTPSFuture.php:280] #4 HTTPSFuture::isReady() called at [<phutil>/src/future/Future.php:37] #5 Future::resolve(NULL) called at [<phutil>/src/future/FutureProxy.php:34] #6 FutureProxy::resolve() called at [<phutil>/src/conduit/ConduitClient.php:64] #7 ConduitClient::callMethodSynchronous(string, array) called at [<arcanist>/src/workflow/ArcanistWorkflow.php:332] #8 ArcanistWorkflow::authenticateConduit() called at [<arcanist>/scripts/arcanist.php:356]
So for newer PHP versions we would need a newer or a patched arcanist version.
I've built a newer arcanist version locally against the stable branch. I had to remove all the patches for the build process to work but at least arc diff works. If wanted I could make a diff for my locally made changes, but someone more experienced with arcanist than I would need to finish it.