Himars relies on inertial navigation the entire flight and uses GPS updates to course correct. If the GPS is blocked for a sufficient amount of flightime, even with the intertial navigation, the accuracy can become unusably low.

This is how the Russians have been throwing double digit percentages of launches off course.

Terminal guidance since ~1995 on higher-end weapons has switched to hybrid inertial + scene matching (various sensor types).

F.ex. the 90s Tomahawk used terrain contour matching to orient itself

For more details see https://apps.dtic.mil/sti/tr/pdf/ADA315439.pdf (US translation of a mid-90s Chinese survey of the guidance space, but it covers the material and is publicly available)

Afaik, most modern systems use infrared target matching for final course correction. (Initially developed to allow anti-shipping missiles to autonomously prioritize targets, but now advanced enough to use in land scenarios as well)

I don't think ATACMS or GMLRS missiles have any terminal guidance apart from their aim point. The GMLRS missiles that carry german SMART munitions do, technically, since the SMART munition has it's own targeting system.

It wouldn't make much sense to me, as most ATACMS warheads are area based, not point target based, so they wouldn't expect to aim at a single target. Also these systems are relatively cheap compared to things that DO have such guidance

They don't and are definitely at the dumb/cheap end of the scale. (or in ATACMS' case, dumb/big)

I think anything developed before ~2005 that wasn't explicitly anti-ship doesn't have terminal guidance. Cruise missiles maybe/not.

Things after (e.g. SBD2 / StormBreaker) started to, because components were finally cheap and mature enough during the development cycle.

Only cruise missile have tercom. Himars and Excalibur do not have any camera or terrain matching system