The main reasons for receiving an Unnatural Inbound Links Penalty via Google Search Console email is because:
Result of utilizing black-hat SEO tactics
Buying toxic backlinks
Unnatural links (also known as “harmful” and “spammy”)
Negative SEO Attack
Over optimisation of anchor text on inbound links
Google do a good job of ignoring backlinks to a certain level.
But once your website passes a toxicity threshold you will see a huge drop in traffic and rankings with a Unnatural Inbound Links GSC Penalty.