Our SP2013 enviroment is in SP1. I understand its good to keep the environment updated with the latest service packs/patches but due to business requirements we don't do it regularly. Just want to understand if there has been any mandatory service packs/security patches for SP2013 post SP1?
Since SharePoint Server 2013 Service Pack 1, there is no any public or mandatory update. Check the full list at SharePoint Server 2013 build numbers
But there's a CU that depend on old CU, don't worry you will find the pre request instructions at the CU download page.
Although the periodic patching is useful, it's recommended to install a new CU that will solve a specific issue in your farm. you can find the issue fixes also at the download page at "Known issue" section.
It's not recommended to install the latest CU of the current month. if you need to install the latest CU try the current month -1 or -2
Installing CU will require downtime.
Nothing post-SP1 has been mandatory for support purposes. If you open a support case, Microsoft will likely ask that you update to the latest CU. In addition, there have been a significant number of post-SP1 security patches that you should consider; that said at this point I would just install the latest CU as managing that many security patches would be difficult.