Add incompatibility blurb for OpenSSH scp >= release 8.8

• scp defaults to SFTP over SCP protocol by default with these releases.
• The origin/reason for the changes started here: https://www.cve.org/CVERecord?id=CVE-2019-6111
• And a good discussion of what changed: https://unix.stackexchange.com/questions/571293/is-scp-unsafe-should-it-be-replaced-with-sftp/571549#571549
• Incompatibility came to light with the following tickets where the requesters did a recent OS upgrade: https://help.hcc.unl.edu/a/tickets/34925 https://help.hcc.unl.edu/a/tickets/34956

content/handling_data/data_transfer/scp.md

@@ -44,3 +44,53 @@ on the Crane cluster to your current directory, you would use the command:
 {{< highlight bash >}}
 $ scp <user_name>@crane.unl.edu:/work/<group_name>/<user_name>/data.csv ./
 {{< /highlight >}}
+
+### Potential incompatibility with recent versions of scp
+
+{{% notice note %}}
+The scp command when used with shell variable or pathname expanded
+source/target arguments, along with certain recursive copy requests,
+may return unexpected errors compared with older versions of the command.
+{{% /notice %}}
+
+Beginning with OpenSSH release 8.8, the scp client defaults to using
+the SFTP protocol over the legacy SCP protocol.  This change in protocol
+removes the user's shell from completing environment variable expansion
+or wildcard pathname matching on the path component being serviced by
+the remote server side of the transfer, as used in the SCP protocol.
+This may cause scp transfers to return errors that worked with prior
+versions of scp.
+
+{{< highlight bash >}}
+# Recursive copy incompatibility example.
+# <source_dir> is missing from the target path resulting in error:
+$ scp -r <source_dir> <user_name>@crane.unl.edu:/work/<group_name>/<user_name>/
+scp: realpath /work/<group_name>/<user_name>/<source_dir>: No such file
+scp: upload "/work/<group_name>/<user_name>/<source_dir>": path canonicalization failed
+scp: failed to upload directory <source_dir> to /work/<group_name>/<user_name>/
+{{< /highlight >}}
+
+
+{{< highlight bash >}}
+# Shell environment expansion incompatibility example.
+# $WORK is treated as a literal string and not expanded by the remote shell:
+$ scp <user_name>@crane.unl.edu:'$WORK/path/to/file' .
+scp: $WORK/path/to/file: No such file or directory
+{{< /highlight >}}
+
+To restore the prior shell expansion behavior on the remote server side
+of the transfer, add the [-O](https://man.openbsd.org/scp#O) flag in your
+scp invocation to use the SCP protocol.
+
+{{< highlight bash >}}
+# Creates <source_dir> and its contents entirely at the target path:
+$ scp -O -r <source_dir> <user_name>@crane.unl.edu:/work/<group_name>/<user_name>/
+# Uses the remote shell on crane.unl.edu to expand $WORK to the string
+# that is <user_name>'s full "/work/<group_name>/<user_name>" path.
+# Note the single quotes to keep the local shell from where scp is invoked
+# from attempting to expand the $WORK variable:
+$ scp -O <user_name>@crane.unl.edu:'$WORK/path/to/file' .
+{{< /highlight >}}
+
+Details of the change are available at the OpenSSH release 8.8 [Future
+deprecation notice](https://www.openssh.com/txt/release-8.8) section.