What does this mean?
Your site contains HTML tags that are not supposed to be used anymore. Generally, ‘deprecated’ means ‘something disapproved’. In IT, deprecated elements are those elements that are allowed, but not recommended and are being replaced by newer ones.
What to do?
So, a ‘deprecated code’ means that it is not recommended to use as there is a better alternative in that software to use instead. Try to use an alternative.
Find out more, here: https://css-tricks.com/why-do-some-html-elements-become-deprecated/