So I have X4 and all the DLC right now, I see we got thew 7.0 patch, but the base game has vanished off GOG and then Timelines is here, what is it? Is it the base game + all DLC and new content meaning we need it, or don't need it... what's going on?

That said, If timelines is everything why is it only $15?

Timelines is new DLC for X4 Foundations and it requires X4 version 7.0 being installed.
What I have been reading (I don't own Timelines DLC) it brings scenarios what player can play. Via those scenarios you get rewards which then come available to basegame side (sandbox as it's called).

If you don't want play Timelines then you don't need it and can continue playing X4 with rest of DLCs you have. But I assume DLC will anyway bring some new sectors for basegame side what can be explored.

If you want you can check reviews and YouTube videos about Timelines and make your own conclusion do you want keep it enabled or not. But make sure your save cannot contain any gameplay WITH Timelines DLC enabled in case you decide you don't want keep it active. So atleast make sure you have point where return in that case.

By the looks of it, you cannot buy only the base game anymore but have to get the X4 Discovery Pack.

If, however, you already own the base game (with or sans other DLCs) then you can buy the latest DLC as a standalone.

And in that case it's much better option go straight to X4: Community of Planets Edition (2024) as that also includes Timelines + other DLCs (not soundtracks).