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For my dad, the issue continues, but we're a tad too afraid to fresh install.
For me, I have no issues with wifi since El Capitan.
 
I have heard of this wi-fi issue before, but to my knowledge, I do not experience it myself. I run a web server and use a Apple Airport Extreme with my cable modem.

One free app you may want to install, which may still be available on the Mac App Store is "BwanaDik". It places a green indicator in your menubar, and includes a lot of useful information if you click on it, such as your current network status, your internal and external IP addresses, and your connected network interfaces. Whenever you lose your Internet connection, the indicator will turn red.

I find it quite useful.
 
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