In particular, the OS X client has a hard-coded mapping which is not well-suited to some work flows. It would be useful if these mappings were configurable by the user.
For example, some common keyboard layouts have no concept of AltGr. On OS X both Alt keys are mapped to AltGr, which can be confusing for users who are not familiar with this modifier.
This configuration also makes it difficult to Alt-Tab switch when ThinLinc isn't in fullscreen mode, since the local and remote systems interpret the keypresses differently.
Unfortunately, because of the AltGr behaviour, disabling this can have very weird effects that will likely be confusing for users. I would suspect that something like bug 4560 would be required to get rid of this mapping.
(The mapping also has the effect of Cmd+Tab on the client being mapped to Alt+Tab on the server, which should increase familiarity for macOS users rather than reduce it.)
This was noted in the community here:
Note that the use case mentioned is really about being able to send all keys to the session in windowed mode (bug 4660), not specifically this issue mentioned here.