How long will be oneplus 6t supported?

I'm thinking about starting to use LineageOS as my default system. I'm considering buying the OnePlus 6T because this phone is not only supported by LineageOS but also by most other custom ROMs. My question is, how long will the phone remain supported? Is the support indefinite, or is there an end-of-life period?

I also want to know how updates work in such a system. Will I receive notifications and be able to update with a single button, or do I need to reinstall the ROM each time a new version appears? Would updating erase all my data?