cant delete system fonts from the trash

With System Integrity Protection enabled, the OS will only allow you to copy fonts from the system folder. But even as copies, it still recognizes them as system fonts and won't let you delete them.

Or, startup to any other drive/partition you have an OS on. You can delete anything you want from the non-startup drive.

Be careful what you remove. The OS does require a minimum set of fonts to work properly. I keep lists of each major OS release and their minimum fonts here.

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[Edited by Moderator]

The issue is that even though they're copies, they still carry the system's permissions that don't allow you to delete them. They'll be forever stuck on your Mac no matter where you put them unless you boot to another drive/partition.

System fonts can't be deleted, Control-click on each one that you put in the Trash and from the resulting submenu choose Put Back.