Creating a psychologically frustrating situation for users could incline them toward less secure decisions. For example, they try to write a password in Swedish, but your input refuses the character å without explanation. Instead of picking a different good password, they throw up their hands and use password123. You have attempted to create a "more secure" system through obscurity, but once we account for user behaviors, it is in fact less secure.

Valid characters are ultimately discoverable through systematic means. Hiding them is not worth the damage you might do to user behaviors.

Creating a psychologically frustrating situation for users could incline them toward less secure decisions. For example, they try to write a password in Swedish, but your input refuses the character å without explanation. Instead of picking a different good password without å, they throw up their hands and use password123—one that's easily defeated with a dictionary attack.

Thus, you have attempted to create a "more secure" system through obscurity, but once we account for user behaviors, it is in fact less secure.

Valid characters are ultimately discoverable through systematic means. Hiding them is not worth the damage you might do to user behaviors.

For further reading, see "Security and Cognitive Bias: Exploring the Role of the Mind" and "Measuring the Security Impacts of Password Policies Using Cognitive Behavioral Agent-Based Modeling" by Sean Smith et al.