Not a softlock per se, but I took the Waterfront Facility route, and getting into the facility grounds proper (LM_F01, the pipe next to the poleplant) was quite difficult for me with a lack of spears, and attempting to do a blast jump would cause slugpup to fall off the ledge as holding down makes them fall through it. I ended up having to spam the jump button against the wall and miraculously got high enough to get into the pipe.

Past that, a lot of scaffolding in Waterfront Facility (bottom right of LM_C04, left half of LM_D03, top right of LM_C03, likely more) are in gaps in the ceiling in such a way that there's no surface to walljump off, and they're too high up or over water/unstable ground to be blast jumped up to. They can be reached, but it requires using the explosive parry for a tiny, tiny bit of extra height at the apex of a jump, which is really finicky to do, and likely unintended/something players wouldn't think of.

Giving the Estranged a smidge more of an upward boost when using the parry, just enough to better get around their lack of jump height in situations where there's a adult-slugcat-height pole jump, would be really helpful. I assume you're trying to avoid them having something resembling a double jump, though.

In Garbage Waste, GW_B05, the tunnel on the right, the part where it turns mostly straight in the lower part of the room, it's always hard to climb there.

In the GATE_UW_SS, you can get softlocked if you enter from the Exterior without a spear or Grappling Worm. The slugpup cannot reach the upper pole even with the explosive parry ability, and even if you have a spear, climbing up there would be very difficult.