Are common names of substituents accepted by IUPAC?
According to the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book) various prefixes are retained for use in general (IUPAC) nomenclature. However, many prefixes are no longer recommended.


  Trivial, common, and traditional prefixes have always been an integral part of organic nomenclature. However, as systematic nomenclature develops and becomes widely used, many of these prefixes fall by the wayside. Accordingly, each set of IUPAC recommendations contains fewer of these traditional prefixes.


The prefix “isopropyl” for $\ce{(CH3)2CH-{}}$ is retained only for use in general (IUPAC) nomenclature.
For the preferred IUPAC name (PIN), the preferred prefix is “propan-2-yl”.
The prefix “1-methylethyl” may be used in general (IUPAC) nomenclature.

The prefix “sec-butyl” for $\ce{CH3-CH2-CH(CH3)-{}}$ was still contained in the 1993 recommendations but is no longer recommended as approved prefix.
For the preferred IUPAC name (PIN), the preferred prefix is “butan-2-yl”.
The prefix “1-methylpropyl” may be used in general (IUPAC) nomenclature.