Minnesota's own guidance on ice is blunt: no ice is ever completely safe, thickness varies enormously across a single lake, and published thickness figures are minimums for clear, new ice rather than guarantees. Ice near inlets, outlets, aeration systems, bridges and springs is thinner than the surrounding sheet, sometimes dramatically so, and it looks identical from twenty feet away.
A Welsh Springer approaching a lake in late November is not evaluating any of this. It sees the place it swims, and if there is open water at the far edge — which is common precisely where the ice is weakest — it sees a reason to go. The distance between a dog on solid ice and a dog in water it cannot climb out of can be a few strides.
So the rule is seasonal rather than permanent, and it is short. From first freeze until the ice is genuinely established, and again from the first thaw until it is gone, keep this breed on a lead anywhere near a frozen lake. That is roughly six weeks in autumn and six in spring. For the deep-winter months in between, follow local guidance about which lakes are being used and where.
It is worth being direct about the other half of this risk: the most dangerous thing about a dog going through ice is an owner following it. Every winter, somewhere, someone does. Call emergency services rather than going out. The dog's chances are considerably better if there is still someone on the bank.