  The dominant mechanism and time scales over which stripped-envelope
supernovae (SNe) progenitor stars shed their hydrogen envelopes are uncertain.
Observations of Type Ib and Ic SNe at late phases could reveal the optical
signatures of interaction with distant circumstellar material (CSM) providing
important clues on the origin of the necessary pre-SN mass loss. We report deep
late-time optical spectroscopy of the Type Ib explosion SN 2004dk 4684 days (13
years) after discovery. Prominent intermediate-width H-alpha emission is
detected, signaling that the SN blast wave has caught up with the hydrogen-rich
CSM lost by the progenitor system. The line luminosity is the highest ever
reported for a SN at this late stage. Prominent emission features of He, Fe,
and Ca are also detected. The spectral characteristics are consistent with CSM
energized by the forward shock, and resemble the late-time spectra of the
persistently interacting Type IIn SNe 2005ip and 1988Z. We suggest that the
onset of interaction with H-rich CSM was associated with a previously reported
radio rebrightening at ~1700 days. The data indicate that the mode of pre-SN
mass loss was a relatively slow dense wind that persisted millennia before the
SN, followed by a short-lived Wolf-Rayet phase that preceded core-collapse and
created a cavity within an extended distribution of CSM. We also present new
spectra of SNe 2014C, PTF11iqb, and 2009ip, all of which also exhibit continued
interaction with extended CSM distributions.
