Custom HTTP Headers with old proxies

Is it true that some old proxies/caches will not honor some custom HTTP headers? If so, can you prove it with sections from the HTTP spec or so开发者_开发百科me other information online?

I'm designing a REST API interface. For versioning I'm debating whether to use version as a part of the URL like (/path1/path2/v1 OR /path1/path2?ver=1) OR to use a custom Accepts X-Version header.

I was just reading in O'Reilly's Even Faster Websites about how mainly internet security software, but really anything that has to check the contents of a page, might filter the Accept-Encoding header in order to reduce the CPU time used decompressing and reading the file. The books cites that about 15% of user have this issue.

However, I see no reason why other, custom headers would be filtered. On the other hand, there also isn't really any reason to send it as a header and not with GET is there? It's not really part of the HTTP protocol, it's just your API.

Edit: Also, see the actual section of the book I mention.