How to access a blocked channel?

The easiest way - web.telegram.org

Settings - Show Sensitive Content - toggle switch.

Telegram Desktop:

Settings - Privacy and security - Sensitive content (Disable filtering) - turn on the switch.

Restart the client.

Switch is available only in clients from the official site. Not available in the client from the App Store or Microsoft Store.

Also, the toggle switch may not be available in some countries - register a sim of another country.