The world is a big place, you don't specify your area of interest. Or what sensor and platform you are using, or even what the target is - so it is difficult to assist. Also, most DEM / DTM / DSM ( and there are important differences between those ) are composite derivative products from from many different sensors and some collected over decades ( like https://lpdaac.usgs.gov/products/nasadem_hgtv001/ ) - so a DEM published in 2018 doesn't mean the data for it was collected in 2018. So I'm not sure what you mean by 'historical 2018 DEM'. If you need some sort of high resolution elevation data product as a reference or back ground, the USGS 3DEP ( https://www.usgs.gov/3d-elevation-program ) and British Ordinance Survey have open access lidar point clouds and derived products like DEM and DSMs. I know for the US, here in the US we have lots subsidence zones ( slopes of active volcanoes, landslides, depleting aquifers, botched construction, failing damns, etc. ) under constant monitoring, some by NASA's airborne SAR ( https://airsar.jpl.nasa.gov/).

The folks at the Alaska Satellite Facility ( which downlinks, processes, archives, and distributes remote-sensing data to scientific users around the world ) are very helpful ( https://docs.asf.alaska.edu/vertex/sbas/# ). The ESA Sentinel program has a similar program.

Dear Michael John Patrick , thank you so much for your detailed response to my question. I sincerely apologize for not providing all the necessary information. Your feedback is incredibly helpful in addressing my specific inquiry and in guiding me on how to frame my questions more effectively in the future. I truly appreciate the time and effort you took to share your knowledge and resources, which will undoubtedly assist me in my research.

Thanks again for your valuable support!

USGS website