Apparently the previous workarounds for 10.15 of manually installing command line tools do no longer work as of macOS 10.15.4, because the iToys company put in some more work to make sure that solution is properly broken on their trainwreck operating system as well. If they only would put half as much time into fixing their OS as they do screwing over their dearly paying customers…
It is weird though that a "brew install"ed subversion would not be usable? Did you type “brew install subversion”, or truly “brew install svn”, because the former seems to be the right one? Our DownloadPsychtoolbox.m script specifically probes if a /usr/local/bin/svn executable exists and tries to use it, without you needing to modify anything.
Can you “ls -l /usr/local/bin/svn” and maybe “more /usr/local/bin/svn” to see if that is actually an executable, and see what calling /usr/local/bin/svn does? And if “which svn” points to the same location? It sounds to me as if somehow svn is redirected to Apples broken svn proxy executable?
Please provide the info requested in this post, and probably do the manual download suggested there for now: