Stripped-envelope supernovae (SESNe), including Type IIb, Ib, and Ic supernovae (SNe), originate from the explosions of massive stars whose outer envelopes have been largely removed during their lifetimes. The main stripping mechanism for the hydrogen (H) envelope in the progenitors of SESNe is often considered to be interaction with a binary companion, while that for the helium (He) layer is unclear. We study the process of the He-layer stripping in the progenitors of SESNe, which is closely related to the origin of their diverse observational properties. We conducted photometric and spectroscopic observations of the Type Ib SN 2021efd, which shows signs of interaction with H-free circumstellar material (CSM). At early phases, its photometric and spectroscopic properties resemble those of typical Type Ib SNe. Around 30 days after the r-band light curve (LC) peak, until at least ~770 days, the multi-band LCs display excessive luminosity compared to regular SESNe and at least three distinct peaks. The light curve evolution is similar to that of SN 2019tsf, whose previously unpublished spectrum at 400 days is also presented here. The nebular spectrum of SN 2021efd shows narrow emission lines (~1000km/s) in various species, such as OI, CaII, MgII, HeI, [OI], [CaII], and [SII]. Based on the observations, we studied the properties of the ejecta and CSM of SN 2021efd. Our observations suggest that SN 2021efd is a Type Ib SN interacting with CSM with the following parameters: The estimated ejecta mass, explosion energy, and 56Ni-56 mass are 2.2M_{sun}_, 9.1*10^50 erg, of 0.14M_{sun}_, respectively, while the estimated CSM mass, composition, and distribution are at least a few times 0.1M_{sun}_, H-free, and clumpy, respectively. Based on the estimated ejecta properties, we conclude that this event is a transitional SN whose progenitor was experiencing He-layer stripping at the epoch of the explosion, and was on the way to becoming a carbon-oxygen star (as the progenitors of Type Ic SNe) from a He star (as the progenitors of Type Ib SNe). The estimated CSM properties suggest that the progenitor had some episodic mass ejections with the rate of ~0.005-0.01M_{sun}_/yr for the last decade and slightly smaller before this final phase at least from ~200 years before the explosion, for the assumed CSM velocity of 100km/s. For the case of ~1000km/s, the necessary mass-loss rate would be increased by a factor of ten, and the timescales decreased by a factor of ten.