Google (who owns YouTube) has always had a function to restrict mature content if there's no user account configured. If you are watching YT in guest mode, they have no idea who you are, so they block such content. But that isn't anything new. As @optikhog mentioned, that's not something that Roku is controlling. It's completely under Google control.
If you want unrestricted YT access, you must log into YT with a user account that they have verified to allow mature content. It sounds to me like you were logged in previously, but for whatever reason the app logged you out. Since it's doing it with your Fire Stick, that should be another clue it isn't Roku doing it.
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