I'm still running this on my OnePlus 6T, purchased refurbished from ebay for $60. Runs fine. Using it degoogled, I'm not sure if e/OS or similar alternatives have any advantage.
It can run PostmarketOS as well which I might play with at some point.
The 6t allows bootloader locking if I sign it with my own keys, but I haven't tried that yet.
A shame it is less supported on newer devices, but these older devices meet my needs perfectly anyway, so I'm not too worried. People buying a new $800 phone every year are just wasting their disposable income IMO.
Maybe you care less about your software security and data on your phone?
The OS still gets security patches up to date, and as far as I know there are no outstanding issues in the firmware at the hardware level. If there were, it would likely take a state actor or someone with expertise to target me, and I just don't consider that to be that likely.
At the moment, a 6t with Lineage is better than the latest Android on a newer device IMO, because of the control it provides. Eventually, I'll switch to something like PostmarketOS, or maybe a Librem or other Linux phone.
For the moment, from a practical perspective, balancing price, performance, control, freedom and security, I don't think the 6t and lineageOS can be beat.