Are the longer patch leads still usable for Ethernet?

Yes, but a long stranded patch lead consumes a substantial part of the channel allowance.

That matters for the 35m, 40m and 50m standard leads and especially the 100m external lead.

Judge the cord as part of the complete end-to-end channel rather than assuming that a 50m lead automatically leaves another 50m for other cable and connections, or that a 100m cord can simply have more patching added beyond it.