Wifi says weak connection on my IPad Pro 2, iPhone work fine along with a computer (Microsoft) this since last update.

Your iPad is no less secure today than it was prior to the update to iPadOS14. However, subsequent to this update, your iPad is now better able to both detect and [warn] bring to your attention security deficiencies of your WiFi network and/or iPad settings. “Working fine” and “working securely” are somewhat different!

This support page will describe the more secure (recommended) settings for WiFi:

Recommended settings for Wi-Fi routers and access points – Apple Support

You may find that your WiFi Router is too old to be (or not capable of being) configured for more secure settings. As such, you can either configure the most secure settings that your Router will support - or replace your Router with something more advanced that provides better security configuration options. 

Whilst WPA3 is the newest WiFi security standard, you should aim to use WPA2 as a minimum.

I think you’re missing the point. Nothing is broken.

Having been updated to iPadOS14, your iPad is simply providing an informational warning that the setup your WiFi router, or the WiFi settings on your iPad, would provide better security is they were updated to stronger security settings.

As for reverting to the earlier version of iPadOS, this isn’t technically possible.

You’re still missing the point. You don’t need to restrict use of your iPad in deference to another device. The security of your connection hasn’t changed for any of your devices; you iPad is now, having been updated, simply able to alert you to a security setting that affects all your devices.

If you investigate/update the WiFi settings of your ISP Router, you may be able to resolve the warning that appears on your iPad.