mruwnik’s answer is correct on the surface level but doesn’t connect it up to the specific use. The specific word “retvrn” references particular Internet waves of rejection-of-modernity sentiment (possibly especially in reactionary politics, though how true this is seems to be contested) that use Roman iconography to represent their imagined idea of traditional Western society. People with such a sentiment often advocate for the reinstatement of stricter social mores that they believe produced better societies in the past—thus the use in the post, where the hypothetical person believes that “our problems started when the fence was removed”.
mruwnik’s answer is correct on the surface level but doesn’t connect it up to the specific use. The specific word “retvrn” references particular Internet waves of rejection-of-modernity sentiment (possibly especially in reactionary politics, though how true this is seems to be contested) that use Roman iconography to represent their imagined idea of traditional Western society. People with such a sentiment often advocate for the reinstatement of stricter social mores that they believe produced better societies in the past—thus the use in the post, where the hypothetical person believes that “our problems started when the fence was removed”.