Marble window sill in the bathroom - how to remove?

I would say the panel is only glued to the console. Mortaring it at the back edge cannot be completely ruled out. I hardly believe it is channeled into the masonry, rather not into the plaster. I wouldn't be surprised if it was simply butted against the original tiles. Possibly additionally "secured" with small pins where the consoles have screw holes (?)

The console itself is likely dowelled and screwed into the masonry (shell construction, so before plastering). That is, its vertical legs, which I estimate to be about 40 to 50 cm, if they exist at all. If the console arms are inserted like arrows (which does not seem unlikely to me here), then I suspect they are embedded about one head deep, so almost fully in an 11.5 cm brick).

Thank you again!

Thanks again!

Gladly, but my hottest tip remains: ask someone who installs such things. This could also be stonemasons or window installers (because it should be similar with window sills), or heating and plumbing installers (who also do a lot of chiseling, I can sing a lament about it after renovating all the neighbors' bathrooms). And of course masons and plasterers, they are also familiar with setting and embedding metal parts in mortar or plaster.

That can help you more than my smartest speculation. I have explicitly stated my profession here in the forum—but unfortunately couldn’t mark it in red.