Hi,
Your understanding is partially correct as you are right to know that current version 5.1 is not currently supported and you might not be able to avail assistance from TAC on the same, however, that doesn't refrain you from putting the workstations in a separate policy and set the policy to install the latest connector.
Please refer to the below article for checking on the supportability matrix:
https://www.cisco.com/c/en/us/support/docs/security/amp-endpoints/215376-end-of-support-announcement-for-amp-for.html
Since you have 70 Windows 7 workstations, I would suggest you test a few workstations by putting in a separate policy and upgrade them via policy. Please note that these upgrades will require a reboot. Please refer to the below article for the same:
https://www.cisco.com/c/en/us/support/docs/security/amp-endpoints/214848-amp-for-endpoints-windows-connector-upda.html
Also, please be aware that Microsoft has ended support on Windows 7 on January 14th, 2020 and it is highly recommended to upgrade your Windows 7 OS to an OS that is supported by Microsoft. AMP will only be supported on Windows 7 if you have ESU (Extended Security Update) from Microsoft for Windows 7 otherwise AMP will work on a best effort basis and any issues with regards to the same won't be supported until unless you have ESUs available with you.
Note: Microsoft ONLY offers support for Win 7 SP1 with ESU.
Please refer below Microsoft articles for more details:
https://support.microsoft.com/en-us/windows/windows-7-support-ended-on-january-14-2020-b75d4580-2cc7-895a-2c9c-1466d9a53962
https://docs.microsoft.com/en-us/troubleshoot/windows-client/windows-7-eos-faq/windows-7-extended-security-updates-faq
I hope the above information is helpful.
Cheers,
Pratham
