I had to read that a few times to make sure I had it correct. What kind of idiot would even think Windows 10 can’t support gigabit speeds? Who makes this stuff up? People who say crap like that should be canned, because people will repeat that bad information. If WIN10 can’t support gig, then what can? I transfer terabytes of data every week across our LAN at work between NAS units and NT 6.1 (WIN7/WS08R2) devices with transfer rates of 110 -115 MBps or 880 - 920 Mbps. Hardly any CAT 6 cabling in the building, mostly all CAT5e with switches connected via a fiber uplink.
If you have a 10/100 NIC on a Windows 10 machine, that would prevent you from getting gigabit speeds obviously. Or if you have any 10/100 hardware in-between end user devices that will do it as well. But Windows 10 has no problem with gig speeds, LAN or WAN.
Discussion of CAT7 can turn into a religious debate. It’s a standard, but not a widely recognized standard. I have no problem getting near gigabit LAN speeds out of 200’+ runs of CAT 5e. Whatever ones view on CAT7 is, I highly doubt that was the reason for your improved speeds. Whatever your previous cable was might have been bad. Especially when it comes to shorter runs, with gigabit it doesn’t matter if it’s CAT5e or CAT8. If in need of longer runs that support 10GigE are needed then going above CAT6 makes sense, unless you want to go 100% fiber. And prices are going down. I think I’ve seen an Intel SFP+ NIC for 10GigE for $150 on NewEgg.