I know LG runs on SMF, specifically version 2.0.19, which technically is not archived yet but I digress. The most recent version of SMF is 2.1.6, which adds two-factor authentication support and IPV6 support, but of course the biggest addition is primarily the ability to draft messages, so like Gmail, you can save the draft and resume writing it later.

I am mainly asking to know what the effects on making this upgrade would be. Would anything get deleted? And how would the site get changed overall?

My concern will be that many "modern" web apps have extensive "user experience", which make user experience into a misery once the Javascript support on the client is not up to what the web designer with the latest version of Chrome expects

Any use of AJAX at all, or any similar framework, is an immediate write-off

I am not familiar with SMF later versions as a user, and not with any versions from the admin side, so can't say much about whether new SMF is good or bad in this regard

Many years ago i have instaled a couple small forums powered by PunBB/FluxBB, those were (at least then) well performing, usable, "no UX" forum systems. I have not followed if anything changed since

However, converting the forum to another system is a major way to lose data, metadata, break links in posts, break external links to pages in the website and so on. I would not do this unless absolutely necessary