"Allowing a domain name to expire presents several risks, including potential cyberattacks, reputational damage, and the loss of control over your brand's online presence. Cybercriminals may seize the expired domain and use it for phishing, malware distribution, or other malicious purposes. Additionally, a new registrant could impersonate your business, leading to customer fraud and trust erosion."
-Google Gemini
Your expired domain may also be used for subjective, adult content.
Example of client domain that expired and used for nefarious content.
Another trick is to use the WayBack Machine website to grab a copy of your previous website, recreate your website, and post links to third-party sources for link farming (a method of gaming the search engines for higher page rank).