The legality of using static residential proxies/datacenter proxies depends on how they are used. If you just doing SEO&SERP, marketing
For example, what do you think of web scraping? Is it legal?
If your use cases of proxies are web scraping, the legality is mostly based on your targeted URLs. If you scrape public data from some titan platforms, like Facebook, Instagram, LinkedIn, or TikTok, that's legal. But if you use techniques to scrape data from private websites or private personal homepages on these websites, that's illegal.
So it's crucial to judge Static Residential Proxies/Datacenter Proxies critically.