That hasn't been true on switched networks in probably 20 years or so.

Isn’t that only relevant for network topologies that rely heavily on broadcasting to multiple nodes. Eg token ring, WiFi and powerline adapters?

For regular Ethernet, the switch will have a table of which IPs are on which NIC and thus can dynamically send packets at the right transmission protocols supported by those NICs without degrading the service of other NICs.

We have switches now, hubs just don't exist anymore. Switches are not affected by some devices having a lower speed.

Is that really true? If so, is there a saner way to handle this than upgrade all the things to 10GBE? Like a POE ethernet condom that interfaces with both network and devices at native max speeds without the core network having to degrade?

That is complete nonsense and not how switched networks work.