Sophos has recently updated the documentation for 9.4 (and this applies to prior versions as well) and it now includes the following (underline added, for emphasis):
Note – Any modifications done by root will void your support. Even users not logged in as root have direct access to a lot of information on the UTM and should be considered privileged users. Therefore, it is strongly recommended to grant SSH access only to administrators in WebAdmin. For any configuration change, use WebAdmin instead.
To elaborate: loginuser (and likely any other shell accounts on your UTM) has access to far more information and configuration capability than most administrators expect.
Sophos considered this a documentation issue and not a security issue in our exchange of emails.
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