Connecting to devices running ancient Cisco IOS versions can be a pain as newer ssh clients are not by default supporting weak ciphers and older key exchange algorithms as these old devices only support insecure ciphers.
I had an alias in my profile for this long ago but after upgrading my OS I ran into the issue again, in this case it was the following version of IOS:
Cisco IOS Software, C3750 Software (C3750-IPSERVICESK9-M), Version 12.2(55)SE6, RELEASE SOFTWARE (fc1) c3750-ipservicesk9-mz.122-55.SE6.bin
I found my previous workaround but this was no longer working, the fix was to also specify the cipher, adding the below as an alias in my .bashrc profile:
alias ssho='ssh -oKexAlgorithms=+diffie-hellman-group1-sha1 -caes256-cbc'