Serial is fully compatible with El Capitan (OS X 10.11).
El Capitan includes an important new security feature called System Integrity Protection, or "SIP". This feature prevents programs from modifying certain protected areas of your machine including the System directory, even if you have administrator privileges (or "root") on your machine.
Unfortunately, this means if you have certain legacy drivers installed in your /System/Library/Extensions directory when you upgrade to El Capitan, these drivers will be removed. Third party drivers should never have been installed at this location to begin with, but some device manufacturers have ignored Apple's guidelines for years. To make matters worse, these certain manufacturers also instruct users to disable System Integrity Protection and Gatekeeper, two important security features, to get their driver working under El Capitan. This leaves your machine vulnerable to several attacks, including but not limited to malicious drivers claiming to be from a device manufacturer but in fact are not.
Serial will continue to function normally regardless, however, because it includes built-in support for these affected devices that works regardless of what drivers may be installed on your machine. Serial uses "user-space" programming interfaces that operate within the security sandbox and does not require you to disable any security features provided by Mac OS X.