"First released in October 2013, and updated several times since, the DOCSIS 3.1 suite of specifications support capacities of up to 10 Gbit/s downstream and 1 Gbit/s upstream"

https://en.wikipedia.org/wiki/DOCSIS

I wonder what speeds would be achievable over the ancient copper wire in my building.

Depends. If it's 50+ years old it'll be fine, sure, some of the joints might not be, but that's not awful to fix.

Then Copper prices went up massively for a while and thinner guage twister pair went in at best, and Aluminium was used at worst.