I am receiving complaints from people unable to connect to servers that they used to be able to using putty on versions of Windows prior to Windows 10.

     The reason you are unable to connect is that as exploits are found in various encryption schemes, those schemes are retired in favor of newer more secure encryption algorithms.

     All versions of Windows prior to Windows 10, with the exception of Windows 8.1, are past end of life.  If you are using one of these, you will not be receiving updates that update security issues like encryption vulnerabilities and thus you will not be able to connect with your old encryption methods.

     Putty is an aftermarket ssh client for older version of Windows.  If you have Windows 10, you do not need putty as it comes with openssh, you just need to install it as one of Windows features (it is not installed by default).

     If you are using 8.1, you should upgrade your putty installation to the July 21, 2021 release.  If you are using an older version of Windows you can try this as well but older versions may not have the necessary libraries for it to work properly.