Hi,
The plugin is fully compatible with wp 5.6.
That is only an informative deprecation warning for a yet existing object reference. The plugin checks the existence of the $.browser object (firefox backwards compatibility check), which triggers the warning in the jQuery migrate script, to inform developers. This attribute will be removed once the deprecation period is over. And even then, not removing these lines will not cause any issues, as there is an existence check on the $.browser attribute.