Deprecated code is a problem which you should not normally see in a current and well maintained website. It is a sign of age and neglect. What is it? Here is a wonderful explanation from ‘TechTerms.com‘.
The short version is this. Programing languages change over time. What currently works is different from what worked some years ago. As the languages change some old code is rendered obsolete and replaced by new code. Or, is not required because of parallel developments of it’s framework and/or the environment it works with. When this happens, it is deprecated, i.e. rejected and no longer works.
You may be having problems for which you don’t know the cause or maybe your website is running slow. You can utilize browser tools for developers and see the errors if they are there. When you do, it is time to replace and update the code.
Regular maintenance and updating should be a part of your overall website care. Along with malware protection, SSL Certificates, regular backups, and ongoing upgrades. These and new content are often done along with your SEO efforts to keep your website keyword rankings high (show up near the top in search results when someone does a search for your website, your products or services, or other when looking for what you are offering).