Adiabatic multimode Y-junctions are extensively used for the design of multimode silicon photonics devices due to their straightforward principle in achieving mode splitting and manipulation. However, experimentally, the limited feature size achievable by lithography can greatly deteriorate the adiabatic branching property and result in large excess loss and crosstalk. Here, we propose a branch structure consisting of a circular-hole-based chirped subwavelength slot. By optimizing the holes' radii and positions under our fabrication constraint, we designed and experimentally demonstrated both symmetric and asymmetric approximately adiabatic four-mode Y-junctions with excellent performance close to ideal Y-junctions over a broad wavelength span. Such junctions are compact, CMOS-compatible, exhibit a relatively large fabrication tolerance, and could be extended for more modes; hence they can be potentially deployed for mode-division multiplexing silicon-photonic systems.