Is the Macbook Pro limited to about 800Mbps of 802.11ac Wifi? I have a 2014 Macbook Pro and have a 2020 Macbook Pro from work.  If I Option Click  on the Wifi icon on the taskbar, they both showed 802.11ac and Tx of 780Mbps or so most of the time. One time it showed 1170Mbps but it only appeared for a second.

And if I copy a file over to another computer on the wired LAN, it is about 70MB/s so it seems 780Mbps is about right.

Is it true that the Macbook Pro's 802.11ac Wifi is limited to about 800Mbps? 

I guess 800Mbps is already quite close to the limit of Gigabit, so if at home we only have Gigabit Ethernet and switch, then 800Mbps is close to the highest speed already. It only would be if at home there is a 10Gbps Ethernet and switch that we only need to be concerned about higher speed.
No, there isn't such a general 800 Mbps limitation.

The theoretical limits of the WiFi in those two MacBook Pros are 1300 Mbps. For example the 2014 MacBook Pro uses a BCM4360 WiFi chip from Broadcom that allows for speeds up to 1.3 Gb/s.

In practice, the actual speed is limited by factors such as distance from the access point, obstacles between laptop and access point, reflections of the signal, other interfering signals, the WiFi hardware and software support of your access point, traffic from other devices on your WiFi, etc.