Hi Paul 
These are not errors, but rather warnings. They won’t prevent your site or some feature from working.
They may come from Discourse, theme/components or plugins which have parts of code that don’t use new methods or variable names (but the older ones are still working). Nothing to worry about.
Just remember to keep your forum up-to-date from time to time (these messages will go away at some point in the future when the code is updated)
edit: it was also a duplicate of Deprecation notices after upgrading to 2.9.0beta10 - #2 by pfaffman