Are there are any plans to eventually backport the 5.0.0 changes to older major (or maybe even minor) versions of Minecraft?

I initially tried doing that with 1.19.2 but the amount of changes that Minecraft did that version made it really annoying. Regarding the other major versions. By downloads, 1.18 accounts to 1.78% of DashLoader downloads, the overhead in maintaining that version heavily outweigh any potential benefits of supporting that version.

So sadly I don't think i will be backporting DashLoader any time soon.