Wifi drops when phone is locked after 10.3.3 update

This is not anecdotal. I have been testing it with every iOS release. Nothing has changed. Here is how to actually test it:

• On your phone go to Settings/Wi-Fi and tap on the i next to the network name
• Write down the IP address
• On your computer open a Terminal window (Mac) or CMD window (Windows)
• Enter "ping <the IP address you wrote down>" (Mac) or "ping /t <the IP address you wrote down>" (Windows)
• Note the phone responding to the ping command
• Press the SLEEP button
• Watch as the responses stop after about 30 seconds
• To stop the pings enter <CTRL>+C.

Every time a major version is released I do this again. The response has never changed in 10 years.

Repeat the test with the phone plugged in; it won't go to sleep. It may wake up for a few seconds if the phone receives a notification such as a Push email alert, which is an external stimulus, but apps that originate communications will not wake it and will use cellular data.