I own a handy asus x00p with android 8 that fits my hand (released in 2018). I have a newer android 11 device (released in 2020) but it’s larger and heavier and I don’t really like using it.
What are the security risks, if I go ahead using the 2018 model to 2fauthenticate?
None if you keep it offline
I’m gonna have to ask you to elaborate:
how can I keep it offline? I need to keep the device online to receive the code, right?
2fa codes are usually time based. As long as the system time on the device is correct it should be able to generate a matching code
Is it not supported by lineage os?
You can use an OTP Generator like Aegis Authenticator from F-Droid. Afaik it even works offline, so there should be no risk if you strictly keep it offline.
Why would an OTP generator even need connection to anything?
Cloud backup or to download images for the services the code links to.
Both are optional with Aegis, I use Aegis also.
If it’s not receiving security patches then it’s not a good candidate to use for 2factor. Risks are low but anything without security patches becomes a minor speed bump to bypass as published exploits will likely exist that are trivial to implement.